If this is for shooting your R/C trucks / cars I'd recommend looking into a 70-200 F2.8 L or f2.8 L IS. Much faster AF & sharper because you can shoot a faster shutter speed thanks to the faster glass.
You can buy either a 1.4X or 2X extender to increase your focal length, but you will lose another 1 stop of light-gathering ability with the 1.4X, and 2 stops with the 2X.
I don't think you can mount the extender with this lens.
You appear to be asking two questions about the 28-135mm... How to get more "reach" and how to focus fast enought to keep up with action photography. Let's consider the focusing first.
The 28-135mm is a USM lens and in decent light is plenty fast focusing for action....
This is a "walk over" Trail Trials obstacle... But some horses prefer to take it at a full gallop and treat it like a jump! They look good, but lose points!
on a 7D, the lens is plenty quick enough to keep up with a horse at full gallop. In fact the 28-135's AF speed is pretty much equal to that of a 70-200L or 300L.
Yes, one technique is to use One Shot, prefocus on a spot where you want to take your shot, then wait for your subject to arrive there. You just have to time the shutter release perfectly.
Alternatively - and usually best - put the camera into AI Servo, use Single Point focus and select the center AF point, then start tracking your subject well in advance and take shots of it along the route. You an trip single shots or set the camera to continuous shooting and take short bursts. It's usually easier to time single shots, but bursts can be fun, too.
Optionally, also use Back Button Focusing, which is a popular technigue among sports photographers.
It takes some practice and work to learn to keep the AF point right on your target. Smaller "targets" such as RC cars can be more challenging. On 7D you have Spot Focus, which provides a smaller AF point. However, it also slows down focus tracking a bit. Still, you might give it a try. I know some folks use it for butterflies successfully and they are difficult, quick moving targets.
On 7D there are a couple more "tweaks" you can use to improve your percentage of in-focus keeper shots in different situations. If you find the AF is being "distracted" by foreground or background objects, due to the small size of your subject, try setting Custom Function III, 1 a bit toward the "slow" setting. This doesn't slow down tracking, it slows down how quick the AF is to jump from one target to another. Leave C.Fn III, 2 set to 0 (default). This might slow shutter release for both the first shot and for any subsequent shots in a burst, because it prioritizes focus accuracy over shutter release. C.Fn III, 3 I usually leave set to 1, Continuous AF track priority. But I think it's mostly concerned with using multiple AF points, so it won't matter if you are using the Single Point or Spot Focus modes I suggested.
It's more to do with what you see in the viewfinder, but I leave C.Fn III, 8 set to 1, Enable... this flashes red in the viewfinder momentarily when focus starts (and gives it as Focus Confirmation when using One Shot). C.Fn III, 9 leave on 0, disabled. This way only the AF point(s) you are using is shown in the viewfinder. C.Fn III, 10, leave set to 0, enabled... so that focus points are displayed in AI Servo and manual focus modes. Finally, C.Fn III, 12, leave set to 0, same for both vertical & horizontal... this allows you to set up different AF points when holding the camera in landscape or portrait orientation.... however it causes a little delay and can be a problem when shooting really fast. I'll use it in studio for portraits, but not shooting sports/action.
If you have a "protection" filter on your lens, remove it for best focusing performance and sharpness. And if you don't have one already, get the correct lens hood and use it... it might help focus a bit, certainly can't hurt... and might protect the lens from bumps and bruises, too.
When using the 28-135, I pretty much always am hand holding it, so I leave IS on all the time. (If using this particular lens locked down on a tripod, the IS needs to be turned off.)
If using it all the way out at 135mm, I recommend stopping it down to f8 whenever possible. Only at 135mm focal length setting, It tends to be slightly soft wide open. It's fine wide open at other focal lengths.
But you also sound as if you want more reach. Well, unfortunately there's no practical way to do that with the 28-135.
Basically you need a longer telephoto lens. Personally I use 70-200/4 IS, 70-200/2.8 IS, 300/4 IS, 300/2.8 IS and 500/4 IS, all of which are excellent for sports photography. Some of these work well with teleconverters, too. I don't have either of them, but some people really like the EF 400/4 DO, 400/5.6 and 100-400mm lenses too. I know folks who use certain of the EF 70-300mm lenses for sports, too.... the ones with USM focus drive are quick enough to keep up with moving targets (70-300L, 70-300 DO, and 70-300 IS USM). There are also some pretty good third party lenses: Sigma 120-400 and 150-500 are a couple popular ones (with HSM and OS, which are similar to Canon USM and IS).
You sound like, and your site looks like, you are a guy trying to get professional looking photos. But you are using a so-so lens.
You have a great camera that is capable of amazing pictures.
Of course it can be done with lesser equipment. It can be done with lesser equipment but will be more challenging and you will find fewer keepers.
Top of the line is the duo 24-70mm f2.8 and 70-200mm f2.8. It don't get better than this lens combo. This is what I use when the rubber meets the road.
But there are a few very good examples than can be had for a more friendly price point. The 70-200mm f4 for instance. Perhaps the 24-105mm f4 or 17-40mm f4 for the more wide end.
The problem with off-brand lenses is, they are likely no better than what you have except for aperture maybe. Some are quite horrible so if you decide in that route, TRY IT FIRST!
One exception is the Sigma 50-150mm f2.8. It is a fantastic lens and for the right money. It comes in a 70-200mm f2.8 version too. I have one on my 7D. I highly recommend it. It could use Sigma's 1.4x in a pinch as it is that good. But I don't recommend extenders at all. That is why I haven't mentioned them.
And when all is said and done, you need some PP software. That is where it's at. Lightroom, Photoshop Elements or Photoshop.