Holy old posts batman
To use the airport express without a dac with your glite, take a mini to rca cable and hook them up. You can't add a usb dac to the airport express.
That said, I recind my above comment about the airport express being better than any usb dac I've heard. The various headroom dacs and the pico quite handily beat the airport express. But in the price range, I still prefer the airport express. And you can add an optical dac to it later. The advantage the aex has is it's not electrically connected to the computer, so you get some noise isolation. If you then hook it up to a dac optically, you're golden.