Hi Balazs, if I’m understanding you right then another solution might be to…
Import your image sequence into FCP, creating a new sequence with the matching codec/size/etc. Then add the new audio to those sequences, and then send all those sequences straight to Compressor (highlight sequences, right click, send to, compressor).
Compressor will now encode the merge file for you, just make sure that you have the Audio enabled on your settings.
No double rendering!