using different time signatures in same score.

I have a song that is written in 3/4 time for some of the instruments and 9/8 for others. They overlay properly but it won't let me change time signatures for individual parts. It will let you do that with the key signature but not time. I understand that this is not a usual occurrence but it is used in some music. Let me know if there is a work around so as to make this work.