I'll be frying. Fresh or frozen, always brine em before going into the fryer. Altons simple brine. 1lb brown sugar, 1lb kosher salt, and i add 2 quarts orange juice.
You can brine a frozen turkey as long as it has not beed injected with a sodium mixture, usually marked "natural" at least heredo not brine a treated bird.
You can brine an injected turkey as well but you'd have to figure out your salt content. The whole purpose of brining a turkey is to PUT salt into it. If you already had a salty turkey and brined it with NO salt in the water, it would actually pull salt out of the turkey.
The whole process of brining a turkey creates an equilibrium with the water outside of it.
My 19 pounder and a couple of pies.