If you own the games, you can technically download the image files from the internet. You’re allowed to possess backups of games you physically own 🙂
That’s not true. it’s illegal to download any copyrighted image files off of the internet regardless of if you own it or not, and according to many of the video game companies its also illegal to subvert DRM etc. to make dumps of discs etc. Just because everyone does it, it doesn’t make it legal.
that being said, you can use something like imgburn or other disk burning utilities to create bin and cue files. I’m not sure RetroPie is the best forum for learning how to burn mega CD’s, I’d be willing to bet with a little googling you could find a site that has tutorials on it that are more tailored specifically to your goals.