How do I make the Time Capsule backup a USB Memory Stick?

I have a 16GB MicroCenter USB memory stick(FAT32) that I keep most of my files on. I use it back and forth on both Windows 7 and my Mac(OSX Lion). I have a Time Capsule/Time Machine setup and running that backs up my Hard drive. I want it to backup my USB too, whenever it is plugged into my computer. Under Time Machine Settings, under System Preferences, I found the memory stick excluded from Backup. It is grayed out and will not let me include it.

Anyone have any suggestions? Thanks!