While trying to change my oil, I accidently drained the transmission fluid.?

While changing the oil on a 95 Nissan altima i drained the transmission fluid, now can i just pour more back in, or should i go get my transmission serviced? When I drained the transmission fluid, I didn't pour any more back in because my friend who's worked on cars before told me I just majorly messed up since its a pressurized system. That didn't make a lot of sense to me, I figured i should just be able to re-tighten the bolt on the transmission plate and pour more fluid into it.