The heartbreaking reality is that the old FD and FL series lenses CAN be used with newer DSLRs, but an adapter is needed to convert it to the EF mount used by DSLRs. That will get them properly mounted to the new camera. Some adapters have an internal lens or two to correct for the distance change to the sensor. Some are nothing more than extension tubes with different mounts on each end. Each has it's benefits and drawbacks. Alternatively, there are one or more small businesses that will physically put an EF mount on an FD mount lens for a fee.
The issue, however, is the auto-focusing system within the DSLRs. Without the internal mechanisms of newer AF lenses, you will have to manually focus each of your pictures. The difficulty with manual focusing is there is not a split-ring focusing screen (or any other kind) that comes standard in a DSLR. Several types can be purchased and installed (either by yourself or trained technician) that will solve that problem. Alternatively, using Live View mode, especially with zooming in capability, you can focus quite well...on more-or-less stationary subects. But Live View can't use the split ring focusing screen, as it uses the sensor itself for the image.
Some photographers are more than willing to use an FD-to-EF adapter and manually focus, especially when using long to super telephoto focal lengths. The cost of a 400mm EF lens is higher than most want to pay, so adapting an FD lens of that focal length is a good solution. The same may apply to wide angle or macro lenses. Some photographers have purchased formerly very expensive super telephoto FD or FL lenses and used them with an adapter since the price of those lenses is typically less than 10 cents on a dollar
While it has been bantered around on various forums, a digital camera back for existing film cameras would be a fantastic solution. To my knowledge, no manufacturers have produced any, probably due to a very small 'market' for such a product.
