Underquilts go beneath the hammock. You need them underneath to keep the air space necessary for insulation. Anything on top of the hammock will get compressed and be less effective as insulation. You could have something on top of the hammock for comfort if needed in addition to the underquilt. If it's really cold a hammock tent in addition to the underquilt (& sleeping bag/blankets) would help.
You can rig an underquilt by taking strong clips and attaching a blanket, comforter or sleeping bag to the underside of your hammock. You want it to fit pretty snugly beneath the hammock.