Importing MIDI:
Was BiaB used to create the MIDI file or was it another source?
If BiaB was used to create the MIDI there are various reasons contradicting my answers below, as I am assuming it wasn't. If it was my answers would be different.

If you actually import a 3rd party MIDI file, 'change all BB tracks to regular tracks' in RB so you don't have to worry about generating over them and can handle them as simple MIDI.

Then Save As and select the .mid format. RB will save everything as MIDI. It may pop something up saying Audio will not get saved, but if you are in the MIDI realm that is irrelevant.

As to why you would want to generate anything:
A. you still can, just set the Track Type on a new track and generate there.
B. Why use RB if you don't need to regenerate? Do you just like the editing features in RB compared to Ableton?

I ask because Ableton is one of the few I haven't tried before.

Make your sound your own!
.. I do not work here, but the benefits are still awesome