First issue is if your controller will accept more than two zones. If so, you are in luck and will be about to reuse it; else you are looking at buying a new controller. Relazing that you are looking at a multiple weekend project assuming you are adding some significant piping and heads, it can be a very fullfilling project. Professional install is not as expensive as one would think. Uncle just had a zone added to his place for $200.

I agree first see if your controller has additional capacity, the controller may say right on it how many stations it has otherwise look where the wires attach to see empty spaces on the termianal strip, if not you will need a new one which is reasonably cheap and easy. While your at it look at the field wiring to see if there are already spare conductors between the clock and the valve manifold; look at the terminal strip in the clock and see if there are any unused wires; this will likely make your addition of a zone that much easier.

As for your actual install of the zone, around here at least, irrigation contractors are usually more than happy to come and "pull in" some poly pipe for reasonably small dollars. This is really the hard part and once done it is simply a matter of some easy plumbing and installing of heads and a valve. In my opinion, and not having benefit of seeing your specific site situation, I would consider this to be a simple homeowner project, provided you have some measure of mechanical aptitude and do some reading around the subject at some of the irrigation links provided in other posts.