What you currently have is a combo ADSL modem/router/wifi access point. It handles the connection to the internet (modem), the wireless network, and shuffling data between everything (the "router" part). Only one ADSL device can connect to your phone line, so you can't just slap another one in - which one would manage the connection?
What you can do with your existing modem/router/wifi device is connect a second wireless access point (a similar device, but with no modem or router stuff in it).
Two ways of dong that - the flakiest and slowest is to have the new access point upstairs, and it connects wirelessly to the existing device, and extends the range. This takes up wireless bandwidth and slows down throughput of the entire wireless network, but it means you have one contiguous wireless network. This can be a pain in the arse to setup, depending on the models you have.
The most high performance way (apart from the Apple based solution above) is if you can run a network cable from where your current device is, to upstairs. Then, you simply set up the second wifi access point as a totally separate wireless network. As long as both networks are enough wifi channels apart, this works great - your mobile device will just switch between whichever is strongest usually. If you can run a cable, I can pull a device out of a box, set it up and fire it up to you to just plug on in.
Basically it sounds like I should come visit one day!