Connecting Airport Express to another router

I'm recently did a favour for a friend, setting up their new router (their old one was 802.11g). They also have an Airport express which is connected to the stereo, so they can stream music from their MacBook Air.
I was wondering if it would be possible to connect the airport to their new router, which has a longer range, so that whenever they're connected to the internet via the router, they can stream music to the stereo via the Airport.
I tried to do this using Airport Utility, but it either couldn't find the airport, or it froze.