ryans_supra,
Sorry, but I've only done it by mixing the NS and premixed. Assuming that a whole bottle of additive is in one bottle of the 75W90, you may want to start by adding 1/4 bottle of the additive. The back of the Supra is high enough to slide under and add the goop to the differential without jacking it up (unless you've lowered yours a lot).
Of course, if the bottle of additive is more than what 1 quart of 75W90 gets, you'll have to adjust accordingly. Call Redline and ask them; they've been responsive to my questions in the past.
Since it's so much easier to add slip than take it away, I'd try adding very small amounts. I'd rather climb under there 8 times than drain the case and start over, but that's just me. If you add 1/8 bottle at a time, you'll only need to climb under at most 8 times.
It won't hurt your differential to not have any additive in there, but it will be harder on your tires.
Asterix