I believe Butterball makes both self-basting (essentially meaning brined that they already injected) and fresh or unfrozen turkeys that are not self-basting. It will say on the packaging.
You did not mention if you injection fluid contains salt or not. Myself I would go ahead and inject with the cajun butter. I have heard and read contradictory information as to whether injecting a self-basting turkey increases or exchanges the salt content.
If you have a Butterball I would talk to them. They have a hot line that is open from 8am-8pm Mon-Fri. I hear they can answer any question you asked them about turkeys. Call 1-800-BUTTERBALL. I know it looks like too many digits, but that is the number they give. You can also e-mail them if the lines are too busy; BBTurkeyCo@aol.com
If you do call, let us know what they say.