Adding an upstairs attic space....how to heat it?

Where do you live?
Use a minisplit heat pump:http://www.fujitsugeneral.com/wallmounted9-12RL.htm

Your existing system only has X amount of BTU's/capacity. If you tie into it then you are stealing from other areas and basically it may be undersized. If you upsize it then you have a major expense and the logistics of running ductwork etc. If you want heat only then some electric baseboard heaters are the simplest way to go. And you will have individual control of them which is a lot more comfortable.

I second Yuri on the Ductless (I am a Mitsubishi fan). If your attic space is going to be open for the most part, just go ductless mini-split heat pump and be done with it. It consists of a wall hung air handler piped to an attractive suitcase style outdoor condenser.

If you start "tying into other systems" your head will start to pound, and blood will start to trickle out your ears after you spend all the money to rob from your primary system and then neither area will heat or cool sufficiently:furious:

Feel free to contact me off board and I will be happy to point you in a direction.

Good luck with your project.

Jay