Can you solder a closed valve with pressure on the other side?

No, If you were able to get it hot enough for solder to flow, valve would fly off. Usually you need all water out to get solder hot enough.

You can try. This question has been asked before. I figure the heat from the torch will be sucked up by the water in the pipe making it difficult for solder to run.
But wait! Some of the other experts say they have has success doing it this way. Who am I to disagree.
Give it a shot and good luck, Tom