First, get Reaper setup so you can modify the charts. Then, extract the song files from the RB2 Export Pack (first depack them with C3 CON Tools, then open a single song package to extract the files).
Modify the songs.dta file to give it a new internal song name and preferably a new song id. Then carefully repack it with C3 CON Tools so that the internal folder and file layout match that of the original song package, but with the new internal song name.
This won't work online, however. To do that, look into the Harmonies Project.
All of this assumes use of an Xbox360.
It's also worth noting both videos above show errors in the charts. I charted double bass for several songs, including both above. I could share my midi files with you once you've got it working.