Yes, but you don't want to
You can place other files on there and use it as additional storage, but those files will NOT be backed up as part of time machine, as the time capsule was NOT designed to back itself up. It is only intended to back up files from other drives/computers.
I do not recommend placing other files on there if you need them to be included as part of the time machine backup.
I've personally never owned one, but read reviews that they work great until the power supply dies 18 months later and you are stuck with a brick. Maybe Apple improved them since I last researched this, but I think external drives connected to a Mac mini connected to the airport extreme base station would be the best of Apple wireless storage/iTunes server solutions. Again, I haven't personally tried this yet either. Oh well!