It is entirely possible to enhance your signal from a Linksys N router. You are correct however, in assuming that you need the airport extreme to be able to use the Airport express. Therefore, you need a different type of range-extender that is compatible with Macs.

I currently use the Wi-Fire which attaches via a USB port and enhances the signal of any router since it is a directional range-extending antenna. Just point the antenna in the direction of your router (which can be located anywhere) and click connect in the connection manager. Its really quite a simple device and has the streamlined look/feel of using your normal Mac connection.

The adapter is made by a company called hField, and only costs around $60.

I hope this helps!
- Rebecca