Just to clear things up, while the Express does have a USB port, it is only capable of sharing printers. The HDD sharing is specific to the Extreme and the Time Capsule unfortunately.
You can't partition the hard drive of the Time Capsule without physically opening it up and seating it in a hard drive enclosure as far as I know. If you wanted to do that, your best bet would be to get an Extreme and connect a USB HDD after partitioning it with your Mac (HFS+). If you get a Time Capsule, you can still use the HDD to store additional files, but Time Machine will continue backing up to the drive until it is full, so you will need to monitor the space manually. There are however some that have managed to "partition" the drive by creating disk images through disk utility (not exactly sure how myself, but a search might prove useful) to allocate space to both Time Machine and other files. A third option would be to get a Time Capsule and use it for Time Machine, and also a USB HDD and connect it to the TC to use as a wireless HDD.
