Copying from one file to another

How do I copy from one file to another while preserving the metrical features of the second file?

I have a section (call it "A") in one file in 4/4 time. I want to add onto it a section from another file ("B"), which is mostly in 3/4. If I just paste B onto the end of A it rearranges B into 4/4. I tried creating blank bars at the end of A and marking the first new bar 3/4. That's fine except that B includes some irregular bars in 4/4, so when those are reached, the system rearranges those bars into 3/4 and knocks subsequent bars out of synch.