Yeah you squash it manually, I don’t know about any automated method.
But this is important: using midi still means you have to go through and fix each track.
It’s extremely rare to find midis who are actually 100% to every note. I’m not sure I’ve ever seen one honestly. Even the great ones, even the ones that cost money, will usually have some places where slight variations in the drums/bass/vocals will not be represented in midi. You have to go over the song and fix those yourself.
Even if you did somehow found a 100% perfect midi, there’s still some conversion for RB. For example bends on guitar are represented different on RB than all the midis I’ve ever seen, slides on vocals, etc.