I am converting my garage to a studio, my garage is on a slab so I have to take the air conditioning from under my house into the garage and up to the top plate for above door vents. My question is can I just run duct for this or do I need another fan to force the air up to the top plate.

I would try just the ducts. Be sure to run both a supply duct and a return air duct. If the existing fan can't supply enough air, you could then add an in duct fan. It is possible you may be able to change the leads to the existing fan and speed it up. That would be much easier if you have the installation manual for it.