You could just go into iTunes> Prefrences> Importing
Then choose AIFF Encoder from the Import Using pull down menu. Make sure that you settings for importing into FCE are good. Click the Settings pull down menu and go to Custom; use these settings for use with DV: kHz should be 48.00, 16 Bit Sample Size, and in Stereo.
Then, close the Prefrences window, go back into your song and right click on it. Click on the Convert Selection To AIF (or AIFF?). This will then create another copy of your existing song, except with higher quality, size, and of course in a diffrent format.
Then just import the file into Final Cut Express. But remember to change back your importing format back to the format you want your songs to imported as for later use.