Perfectly doable - put a thin bead of RTV on the shift tower housing before you put the shifter back, make sure the lower shifter lever properly engages the shift rails, and pay attention to the amount you put in! It's easy to over/underfill when pouring fluid in from the top, and both are bad (just in different ways.)
But, I have done this when I don't have a "suction gun" (think pint-capacity syringe) for filling the transmission.
"recon" (sic - reckon)(tm) "hihgly"(tm) "seceed"(tm)
"Outback AIDS - Alcohol-Induced Dizzy Spells"