Yes, two (easy) ways to hookup a powered sub in a 2-channel setup.
1) Run the speaker wire from your amp to the sub, then another set of speaker cables to your speakers. The sub will suck nearly no current from the signal (so your mains will still get almost all the juice), expensive subs have nice crossovers you can use in this configuration.
2) Run a line level signal (rca's or xlr) from your preamp or integrated to the sub. This can be accomplished by using a second pre-out, splitting an existing pre-out (rca splitters) or using a dedicated "subwoofer out" if your preamp/integrated/receiver has one.
Another option is a dedicated crossover, but these can be tricky to work with, and quite expensive. This area is probably best left to those who really know what they're doing and have a very expensive system to justify it.
For a powered sub, SVS, HSU, REL and on the very cheap end Adire (Rava or Rava SE) are all great choices for a 2-channel music setup. There's tons of info on all these subs on AudioAsylum.com and to a slightly less-reliable degree AudioRevew.com.
Good luck!
-dd3mon