It must be possible, however you DO understand that you can't leave the gas burner unprotected and installed. You are talking about 'short' money if all you burn is a ton of coal. It might be advisable to consider a NG boiler as an independent source. The cost to retro, if possible, would not be worth the aggravation. ......penny wise, pound foolish..........plus, I would not even consider NG near a burn pot with an open flame, NO MATTER what safeties it had on it. Do you like the KISS philosophy?
If not, why not?
Let me see....you state that you have NG burning anyway, why do you want to add a burner to the DF???????????