Unfortunately, you can't have A.AUX correspond to any direct single click on the source buttons, but you can easily add it to your "LIST" menu items by accessing the ADD function on the Beo4 (I'm at work, so if I get this wrong, please anyone, correct me).
Press LIST/STANDBY to access the Beo4 Function screen, press UP or DOWN until you see ADD, then press GO, then press UP or DOWN until you get to AAUX, then press GO, and then press EXIT and you're done.
-Sal
Love B&O, but no longer addicted.