Can you add a midi file to a project that you almost finish?

Hi, so I want to add a midi file to a new instrument in a score I'm working on right now. I'm almost done with it, just a few details here and there and that's it!

But, I'd like to add a new instrument (Piano) with some corrections in several parts of the song. That's why I'm asking if I can add midi to a Project and add the instrument, because, I find it easier to just delete the old Piano with the mistakes and leave the new one with the corrections. Can it be done? I mean, adding the midi file.

Do I need to upload a file or something to make myself clear? Anyway, thanks for your time and for Reading! (: