If you get a final film, where all you have to do is to compose the
music for it,
I think you can do it, as long as you're able to play the (digitalized) video
and sync it with your audio-applications.
Render the complete composition with full (film) length as single audio file.
The video guy then should know how to add your music to the final film
and which audio-file-formats his program accepts.
Haven't done it myself however, so this is more a guess.