Checkout if you can get one of the cards to go into master mode and see if you can detect it as an access point.
Decide whether you want to use bridging or routing and consequently you will need to do different things with iptables, brctl and dhcp.
Read howto's as starting point (
www.tldp.org) if you're clueless.
Read documentation about dlink G604T (it runs linux inside) and it will give you ideas on a possible way to acheive what you want to do.
What they are doing is bridging the wired and wireless network together and routing trought ppp link ... so you can get ideas for bothe techiques from there. I mean you might want to bridge together the 2 wifi physically extending the wireless network (since you're not using any of the protocols that do this you will want to di masquerading) or you might want to create another network that is routed trought your Z. Bothe rwquire a fair amount of network knowlege ... if you want to skip anything involving the knowlege I suggest you buy another access point (they are cheep nowdays) and enable the wireless network extension (make sure the wireless network extension is compatible between what you already have and what you buy ... for example dlink is not compatible with 3com as far as the wireless extension ir concerned).
You also might like to investigate if a mesh network could be a solution for you ...