Is this possible?
If so, how?
Configure the Time Capsule to "create a wireless network" and click to ignore the messages that you do not have an Ethernet connection or an Internet connection.
When you want to back up or print, you will need to switch your Mac to the wireless signal on the Time Capsule.
If this is a new Time Capsule and you are making the first initial backup, I would suggest that you connect the Mac using an Ethernet cable to one of the LAN <-> ports on the Time Capsule. The backup will proceed 3-5 times faster than wireless.
No it isn't possible.
Edit.. LOL!! my answer is opposite to BOB.. I assume you want internet at the same time. He is assuming you will swap from using internet to only using the TC in local network and hence removing all internet connections. So my answer is based on how to get them working at the same time.
What you want is two separate wireless networks.. with no links. And to use both networks simultaneously on only one wireless client card. No can do!!
What is possible.
You have to connect the TC to the main wireless router. I strongly recommend this be done by ethenet or an ethernet replacement technology like Ethernet over Power EOP .. the adapters are now cheap and fast enough for wireless link. You will turn off wireless in the TC although you can also set it up with identical ssid and security on a different channel.. then you have one flat network and all devices can roam.
You need two wireless clients in the laptop/computer you use.
You can buy a usb stick compatible with the OS used.. os x although you nowhere state the type of system. USB stick wireless are cheap and plenty are available that will work fine with Mac OS drivers.
On the USB stick, setup manual network with different IP subnet to the main router, and no gateway or dns. This will allow you to communicate to two different subnets at the same time. (On OS X it does not strictly need such care with gateway and dns as you can control the order of devices better than on a pc.. just the same I would do it manually)
Of course you could also just plug in ethernet and do the same thing.
As per Bob definitely do the backup numero uno by ethenet.. it will take ages by wireless and the chance of a corrupted backup is so so much greater in which case it will just start again.