I don’t know how Reaper and the Overbridge plugin work together, but the Overbridge plugin is designed to work in addition to your normal audio interface.
The plugin itself provides all the audio streams of the connected Elektron device(s).
So essentially you “just” need to figure out how to route an audio stream from the Overbridge plugin to a track in Reaper.
There exists already quite a few Reaper+Overbridge threads which may help.
A good starting point is here: