Resolve does not support adding custom IDTs so that they appear in the drop-down with the built in ones. It also does not support CTL directly.
But you can manually convert CTL code to Resolve’s DCTL, and apply custom IDTs using that. I converted the ALEXA EI800 IDT in that way, and provide it via my website at no charge as an example of CTL to DCTL conversion. I also have a few other converted IDTs available there at $30 each, for those who don’t feel up to doing it themselves, but that does not include any Canon ones at the moment.