'lawnboy' has a good idea. Along the same lines, my folks put in a propane gas stove (or you can use a heater or use natural gas w/a fireplace, or any combination) to keep the room warm. As for keeping it cool, there are windows and sky lights. There's no need for AC even though it's on the south side of the house. Oh, there is a ceiling fan that helps circulate the air in the room.
IF you were to simply add ducting for HVAC to the garage room, you may now have an undersized unit(s) for the house. If you want to know for sure, measure the square footage of all the heated/cooled space in the house, and then call the mfgr. of your HVAC units to see what square footage they're rated for. (Maybe the Owner Manuals have the info.)