Although the original poster will probably never read this, it may be useful to others researching this topic.
Yes, it is quite easy to connect your own second telephone line. Usually the wires coming in are red, green (line 1) and black, yellow (line 2).
You just have to connect black to what was red and yellow to what was green on the modular jack you want to convert. Use a pair of wire strippers ($5-10 at a hardware store) or a knife to strip the wire.
NOTE: modular jacks vary; some have screws to attach the wires, while older ones have some kind of crimp to connect the wires. If you have the latter, you'll need to cut the r/g wires and then connect the ones leading from the jack to the b/y wires. Use wirenuts or crimpsleeves -- or better yet buy a newer modular jack so you can switch back and forth more easily in the future.
Another caveat is that if you're in an older apartment building, you may have a bit of a frankenstein phone setup. I've gone out to the box after having argued with Pacific Bell over whether my new line was hooked up -- only to discover that the wire they'd hooked up that supposedly led into my apartment was a short stub that had been cut many years previous and left to dangle in the wall!
In my current apartment, I *appear* to have only THREE wires (3 wires), red, green, yellow, and no black wire. I have no f-ing idea what's up with that. This is the topic the reasearch of which landed me on this page.
In general, though, it is quite an easy job. I would pull your jack off the wall and familiarise yourself with it first, though.
Oh one more thing. Oftentimes on multiple extension homes/apts, you'll have the wire (bundle of the 4 wires actually) in seperate segments from box to first outlet, first outlet to second outlet, etc. It may be necessary to wirenut/crimp the yellow/black wires together inside those other jacks, as they are not usually connected by default.
Hope this helps someone! The email above is only checked once a month or so, but it should be good for a couple more years at least if you have more questions.