I have filled it that way but I think the "preferred" way is to fill it through the fill-level hole on the passenger side of the case.

The grease fitting on the torque-tube "receiver" at the rear of the transmission was not for grease - it was for filling the u-joint ball (pre-filling the ball - the enclosed u-joint receives lubricant from the transmission. I recently removed that "grease" fitting and filled an sm420 through that small hole in the "receiver".