I am using a netgear wn3000 rp range extender outside of my garage,

[csanders0056]

I want attach an old wifi router to the range extender and create an access point. is this possible?

 

[RealBeast]

An access point requires an Ethernet cable connection. What you seem to be wanting is a wireless bridge and it is highly unlikely that you will be able accomplish that with what you have.

Range extenders tend not to work well since they can only work with the wireless signal that they receive and your signal is probably poor outside unless your router is very nearby in the garage.