My house is built on a concreat slab, I have an addition on the back of my house I'm going to remodle. The room has no heat, can I run ducting through my attic to supply heat to my back room. THE HEAT IN THE REST OF THE HOUSE RUNS Through THE FLOOR.

Can you access the supply plenum? If not then it will not be easy and you will most certainly be busting up concrete. Also, how big is the space you are trying to condition. The reason I ask is that if that space was not factored into the original calculations for your system then your system might not have enough capacity to cover the additional area.

Sorry, I just re-read your question. The answer is no, you cannot make run up to your attic. You most likely have a down flow furnace (meaning it pushes the air DOWN to the supply plenum). Trying to push it down and then back up to your attic and then to the space in question is just a bad idea.

Just add some electric base board heat to the new addition. That will make it simple.