Yes, that’s exactly what I did. And yes, that is an .ac3 file, dragged into the timeline:

So here’s my workflow:
My Sequence settings are for 6 channels, all set to 0 db (I had left them at the defaults, but changed them when all this din’t work to see if it would help), all set to Dual Mono.
I then set each audio channel from 1-6 in the timeline. I then exported the tracks you see as the first set in the timeline as Audio as Aiff, using 48k, 16-bit, Channel Grouped. I got the resulting files which I then checked by importing and placing them on the timeline as well (This is the second set in the timeline).
I then open Compressor, and select Add Surround Sound. I add all the 6 exported .aif’s into the window to their corresponding channels, and click ok. I then drag the Dolby Digital Professional 5.1 preset onto the source, and click submit. I then drag the resulting .ac3 into fcp to check the audio, and that is the third set in the timeline.