Burning charcoal in a gas propane grill is not recommended especially if the propane burners are working and in tact.
If you burn charcoal in a propane gas grill then it will damage the propane burners and damage the entire grill.
However if the burners on the propane grill are no longer working or any good anymore then you could use charcoal in that propane grill.
Otherwise it's best to just use the propane for the propane grill or get a charcoal grill made to use charcoal instead.