Unfortunately, not the help that you were looking for.
The AirPort Extreme can "extend" a wireless signal....but only if it has been produced by another Apple AirPort router. It would not be compatible with your DSLl modem/router for this type of application.
Another way of saying the same thing is that you will need 2 Apple AirPort routers if you want to extend a signal using wireless only.
It would be possible to have a single AirPort Extreme provide more wireless coverage by connecting it to the DSL modem/router using a wired Ethernet connection.
If you cannot run the Ethernet cable through the home, you might want to take a look at a pair of Ethernet powerline adapters to send the signal over the existing power lines in your home. More info on powerline is here:
http://computer.howstuffworks.com/power-network.htm