AirPort Express extending AirPort Extreme wireless network: can I use WEP and WPA2 Personal?
First you can save you current configuration in Airport Utility.

Then you have two options:
1. If your Extreme support guest network then just set it up as WEP and extend it with Express.
2. Or connect Express with ethernet to Extreme(i.e. just use it as gateway to internet) and setup WEP network on Express.