If they're all in the same wallet then armory will automatically take from each. I don't think you can specify where the fee comes from specifically.
Fair enough - for practical purposes, it doesn't really matter, as long as I can spend it all. As far as I know, where the money comes from is baked into the bitcoin protocol, so that the oldest bitcoins are spent first. So presumably, if the dust transactions are older, they would get spent first. I just wasn't sure how Armory would handle this.