Most jerk seasoning has salt, but not nearly enough to substitute for salting.
Kayb sent me some jerk rub and I still seasoned the chicken pretty much normally in addition to the rub.
Unless what you use is really salty, you'll still need to season.

But yeah, to summarize, season ahead of time and marinate for as long as you prefer. It's not necessary to marinate for a day or more to get the flavor. Even just one hour will give you good flavor.