oddjobbb, there are some hoops to jump through, but whether it is worth it or not depends on the grill.
Specially, NG and LP grills have different sized burner orifices, valves and gas lines/regulators. To do a full conversion, you have to change it all out and as LB's link shows, Weber will not cooperate. If you can find a working propane manifold for the grill in question, everything becomes much easier.
This all presumes that you are looking to do this on a Weber gasser. More details about the grill in question would be helpful.