Can you convert a propane dryer to natural gas?

Yes you can usually buy a kit from the dealer it's a burner orifice and a regulator cap

Actually most appliances come set up for NG and must be convert to Propane, kit is almost always attached to the back of the unit. It is possible that whoever set your for Propane may have replaced the orifice into the card holder on the back of the unit instead of just pitchinh it, most installers put it in the card.