used canoe rack 2004 ford explorer?

Simplest way would be to add two extension bars to the existing rack with just the right amount overhang on each side to handle your canoe. You could actually make them out of wood, notched to fit the rails and attach them to the rails with zip ties. Once the canoe is loaded and tied down the rack itself will be bearing the weight which will keep everything in place. Remove them when the canoe comes off.

Thule makes a canoe rack module that will fit the factory roof rack.