cable modem with adsl

Yes. A pair of powerline adapters act like an ethernet cable.

Cable Modem - Wireless Router - Powerline Adapter (Room Downstairs) - Powerline Adapter (Room Upstairs) - Wireless Router -

Broadband Over Powerlines (BPL), HomePlug, IEEE P1901, etc. have been around for many years. It's used in Europe & Asia, and slowly being adopted in the U.S.A. I've installed & use Panasonic BL-PA100KTA HD-PLC Ethernet Adaptor BL-PA100A units.

The answer is yes, but you'd be better off connecting a simple wireless base-station to it rather than connect a router to another router (your cable modem, if it's like any other cable modem with wireless built into it that I've seen [and I've seen plenty--I install them for a living], is a router unto itself).

If you already have a wireless N router that you'd like to utilize, then that's okay--the best way to do it would be to disable the NAT and DHCP on your wireless N router, and give it and IP that's on a different subnet than the router of the cable modem. This will essentially convert it to the wireless base station (or I suppose more accurately, a wireless switch) that you are looking for, and pass all the routing responsibility to the router in the cable modem.

Just fyi, this is not a direct line to chris.

Copying jWire4's comment, look at the top, COMMUNITY-POWERED QUESTIONS & ANSWERS not to be a dick or nothing but that's what it is.

The power line adapters are the easiest way to go but if you don't have the money to spend on them, you could possibly do a wireless bridge between the 2 routers. It's a pain to set up but it will work if both routers support that function. Also, you don't have a cable modem if you have DSL. What ISP do you have?