DSP accepts QT files right out of FCP, but if the audio file doesn't match the ac3 you already have in the track it won't let you use the QT sound, the movie is fine. Since you only need the black, that's fine--so just export your five seconds of black separately and drag it into DSP, drop it in the track and then your movie, and save exporting the whole project.
I've done this and the only thing is you have to be really careful because this throws off the sync of your project. I was able to line it back up but to save the worry you could just put the same compression on your slug so it has a sound file and save the trouble.