Specifically, i've got an backyard office (with phone line) that can't get a wi-fi reception from the house because of too much interference. Can I just get another server?
I'm not real sure how DSL works. With cable modems, the cable company has the MAC address of your cable modem and that is your only gateway. If you wanted a second cable modem, you pay for the cable company to activate the second MAC address. I run a second router set up as a repeater (range extender) off the first to increase my signal. I don't know how something like that would help you though if you have interference between the two locations. I am assuming with DSL you can plug in at any of your outlets and you said you have one in the second location. If this is the case, then you should be able to plug in a router in the second location without any problems. My suggestion would be to set both routers up with the same SSID name so that you can take computers back and forth (specifically any laptops) and just have the one SSID name so that you won't have to configure the computer for two of them.
If you mean a second dsl modem, I don't think that is possible on the same line. But with a router, you could use upwards of a 325 ft ethernet cable (you'll lose transmission beyond 325 ft) to connect to your office computer.