Q: airport express router

Add another AirPort Express to "extend" the signal to the area that needs more wireless coverage.

Install the AirPort Express about half way between your existing AirPort Express and the back room.

See this Apple document for more information.....particularly the section about Extend an Existing WiFi Network's Range.

http://www.apple.com/airport-express/

Needs to be another Apple router.....an AirPort Express, AirPort Extreme or Time Capsule.

From the document that I linked above:

Just place it in range of your primary base station — an AirPort Extreme, AirPort Time Capsule, or another AirPort Express

You can also connect your Express to the non-Apple router with an Ethernet cable. Follow these instructions, though the screenshots are outdated and not very useful: Wi-Fi base stations: Setting up and configuring a roaming network (802.11 a/b/g/n)

The first graphic and description is the only part of that document that remains useful.