Hi Claes,
My name is Eric Moyers. I am a Network Support Engineer in the Cisco Small Business Support Center.
The short answer is yes it is possible.
What you would do is under advanced routing change the Operation mode to router from Gateway.
Then under LAN disable the DHCP Server.
Setup your wireless SSID and security.
Set the Local IP address to something other than the IP of the main router and plug ehternet cord from your main router into the LAN port of your WAP/WRVS4400N
That should take care of it. If I had an extra wireless router laying around I would probably do this. But if it was between buying an extra router vs buying an access point, I personally would just get the access point.
Hope this helps.
Eric Moyers
Cisco Network Support Engineer
1-866-606-1866