In a word you need a WDS system (Wireless distribution system). I installed a CAT6 based network for a local Steve Jobs school and the guys from Apple then came and installed the wireless system using WDS. So basically it was one giant wifi network, all with the same name and password. They installed 6 wifi access points, each one connected to the main router.
The only thing is that you will reduce the overall bandwidth of the system.
A quick Google will give youmore information.
Another option is the leaky cable system, think of a hose pipe with lots of pin pricks in it.