Insulate room with concrete floor
(Boy, you are a busy bee!!!)
Assuming the walls and possibly the ceiling are insulated (and you'll correct me if they're not), you can use a Natural Gas Stove, a Propane Gas Stove, Electric Baseboard Heat, or have your duct system 'extended' through the wall into the unheated room. (The only consideration is if your existing HVAC system can provide heat to the 'added space'. In other words, you current system may be 'maxed-out' in heating the rest of the house. Adding another room to the ductwork may take away heat from the main part of the house.) Ductwork can be added, possibly, overhead extending through the ceiling from above and DOWN into the room (on the ceiling.) Or, by 'punching' through the wall that separates the cold room from the rest of the house, and making a new duct or converting an existing duct into a 2-way vent. You may need a return duct if the room is NOT always opened to the main part of the house. You can't just heat a 'closed room' w/o considering how you're going to return the 'cold air' to the HVAC system. (It may be an entirely different matter if the room was ALWAYS opened up to the main part of the house.)
You could consider a Warm Floor System too. It would involve removing the carpeting to do the installation, then recarpeting (if that's what you want.) I'd talk to a reputable HVAC Contractor about your options. There are many, at varying costs to install AND operate (over time.) With the price of energy these days, you'd want to look into as many options as you can when it comes to Operating Costs (as well as Installation Costs.)
My best to ya and hope this helps.
Jay J -Moderator
If you come back w/more ?'s, do tell me what you use the room for (or what you plan on using the room for.) Also, tell me if there are 'vents' or ducts installed on the wall the abutts the cold room (either upstairs or downstairs.)