If you want to stick with FCP 4.5, then you’ll have to convert your files from ‘polyphonic’ (more than 2 channels, I assume) to 2 channels each. IOW, You’ll have to make 2 or more files from each BWF file from the field recorder.
If it were me, I’d simply update to FCP5 & QT 7, then you only need to run your audio through Sebsky tools & be done with it.
Arnie
https://www.arniepix.com