I have sent files from FCP to Protools for mixing. FCP has an option to export an OMF file, which will carry all of the audio exactly as it was in your timeline. However, it will not export any filters or volume information.
When your mix is done, the sound editor bounces you a series of .aif or .wav files, one for each channel of the mix. You just have to import each file & add it to your program. If you made your export properly, the head of each audio file should line up with the head of your timeline.
For safety, you should put a “2 pop” at the head & an “end pop” at the tail of your program: a 1 frame visual cue that lines up with a 1 frame audio cue.
Arnie
https://www.arniepix.com