Thanks for the clarification. You can certainly connect a USB drive to the AirPort Exteme, but it will be much slower than an internal drive.
There is an open question about whether Time Machine is supported when you plug a USB drive into the AirPort Extreme.
The owner's Setup Guide mentions that you can do this, but an Apple support document, which was issued about a month after the AirPort Extreme was introduced states that a USB drive connected to the Airport Exteme cannot be used for Time Machine backups.
http://support.apple.com/kb/HT2038
We've asked Apple repeatedly for clarification. but so far do not have an "official" answer. If you plan to use a USB drive with the AirPort Extreme for Time Machine, you might want to try to contact Apple regarding this.