I like to generate multiple takes of Realdrum tracks and pick the best fills. So I would like to be able to generate multiriffs of Realdrum tracks. Or if that's not possible, I would like to be able to quickly copy the Realtrack/Realdrum assigned to an extra track to other extra tracks so that I could fill all of the extra tracks with the same Realtrack/Realdrum and generate multiple takes.

You can load the actual RealDrum source file into a track and set the original tempo, then you can set it to Snap and simply copy and paste the fills where you want them, same with shots n holds.
If you are using Reaper you can generate directly in there and select any fill.
https://www.youtube.com/watch?v=ElQIB5RYLG0