While I don't have any real knowledge of kosher turkeys, I think the point you bring up is what you need to clarify.
If the bird was packed in sale, but no additional liquid was involved, then brining might be advisable, although less salt might be the way to go...
(Think country style ham here - but not to that extreme, it may actually have a reduced moisture content to it if this was the method)
If it WAS brined, then you're good to go.
Although, even if it was brined, you could still do it again to introduce your own flovorings, it's just that the salt amounts need to be adjusted accordingly. If there's a lot of salt in the turkey already, use a low salt brine, and the salinity will equalize over time, and still be effective.
As always, keep the brine cold - but you knew that already.