Well, you have ducted heat so you can certainly add A/C. We can't do justice to what a cost would be. Something like a 3 ton condenser and coil would be about a $ 900 for the equipment, which the contractor would have to charge you about $1800 for to cover warranty , and the labor to install this could run $1500 to $4500. Lots of variable such as how hard it will be to run the copper, and to run the electric circuit.
One of the variable is the furnace. Your furnace may have a large enough blower to support its heat output, but not large enough to support the tonnage of A/C you need. A well qualified contractor will calculate all this, but the point is it could be new furnace time as well.
There should be some contractors in your area who would be willing to take a look and give you some bids. Around here, the HVAC contractors are advertising that they have overbought on systems....maybe in anticipation of new construction or remodel work which went away... and they say they will cut you a deal.
The condenser can go on the roof or outside. If you have neighbors or a HOA, you need to consider that. Also, a crane will be needed to put it on the roof. Add $$$$