Sorry, you are going to lose quality. Mainly because FCP 5 is very old, and doesn't have a nice, full raster 10-bit easy to manage codec. You could use Uncompressed 10-bit...but none of your drives could play that back without serious lag or skipping.
FCP 5...your best bet is DVCPRO HD. That's the codec of choice for that version. It's lossy, but what you have to work with.