Using Time Capsule to Access Internet
This depends on the type of WIFI network that is being produced by your WIFI router, as to whether the TC can extend it or not.  Basically, the best way to find out is to experiment on your TC - fire up your Airport Utility, and in the Wireless configuration option, look to see if it will let you extend rather than create a wireless network, and then see if will find your existing network.

EDIT: I should add that the brand of wireless router is a primary consideration as well. Many brands don't play well with TC, AE, or any other Apple networking gear.