I've seen it can connect to an existing WiFi network and it work fine. It's not an easy setup so I can't tell you much other than it sounds like you need to re-think the setup and even ask Apple.
I have an Airport connected to my speakers, and WIFI internet using a Plusnet router. (OS is Lion.) I recently had hopes that I could stream music off Soundcloud to my speakers, but my computer will only let me connect to either my WIFI internet or my Airport one at a time. I thought Airfoil was supposed to solve this problem so I downloaded the trial version, but it is just the same: only one or the other. I have looked everywhere in Airfoil support and there's no mention that you have to have an ethernet connection. Am I being dumb?????