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.
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.