Can I use an AirPort Express to extend a wifi network that has a Cisco base station?
I would say without doing the research on the specific model (generation) of Airport and the specifics on the firmware on the Cisco device - they may work but it's certainly not guaranteed.

It's almost inconceivable the first part would fail. The express will join almost any network and AirTunes will work as long as it is in range.

The extending is where you want to do your homework if you need that to work as well to not be disappointed with a purchase. WDS is the main standard - so be sure you read up on the Apple WDS documentation as well as whatever Cisco provides.

WDS in and of itself isn't necessarily a bad protocol, but it makes your network much more complicated and much more likely to fail since any one part failing cascades to the rest. Many people (even professionals) have lost days to trying to make WDS work and maintain/troubleshoot it when problems arise.