Unless it would be built in to specific RF lenses, you couldn't have an adapter sit between an RF lens and the mount.
You'll need to look into either circular screw-on filters or the square type with filter holders for the vast majority of RF lenses.
Of course Ricky's response is spot on.
Both Canon and Meike make EF to RF adapters with drop in / adjustable ND's. Creator MonkeyPixels on youtube has done testing and produced content on the meike. vlog#9 I think. Anyway, while I always recommend canon with canon, the meike for the R5 C is pretty cool. The only real drawback (if this is one) is that the dial adjustment doesn't have clicks. It also has 5-8 special effect filters in addition to ND if thats your thing. (fun)
Bay Area - CA
~R5 C (220.127.116.11) ~RF Trinity, ~RF 100 Macro, ~RF 100~400, ~RF 100~500, +RF 1.4x TC, +Canon Control Ring, BG-R10~DxO PhotoLab Elite ~DaVinci Resolve ~Windows11 Pro ~ImageClass MF644Cdw ~Pixel6 ~CarePaks Are Worth It
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.