You can partition an external drive, then use one partition for Time Machine and the other for storage. Obviously the stuff on the storage partition isn't backed up (unless you do something like burn a copy to DVD), but it will do what I think you're looking for. Disk Utility can repartition a disk on the fly, without erasing data, but I'm not positive whether this is an option with a Time Machine drive. Easy enough to open it up and test.
Alternately, you can store files directly on a Time Machine volume, but I only have experience with such a situation with using Time Machine via the network, in which case the files get dropped into a disk image. Not sure what Time Machine does when it's accessing a drive directly and finds files it didn't put there on the disk--might automatically erase them, or treat them as if they were from an older backup, in which case they will eventually be deleted when you run out of space, to make room for new backups.