Can Time Capsule join an existing wifi network...
... and only be used for backups and printing?
This type of setup is not officially supported by Apple.
Having tried this a few times as a test, I would not recommend this type of of setup for most users. The big downsides to this type of configuration are.....
1) Backups are extremely slow...much slower than normal wireless....because the signal must make a "double hop" during backups.
2) The Ethernet ports are not enabled on the Time Capsule in this type of setup, so it is not possible to backup using a much faster wired Ethernet connection....even if you want to.