Putting player data into level.dat?

I'm going to be doing a realm of my friend's server, and he sent me the file to upload. So far, it's fine, except my current character uses my friend's character data. Is there any way I can take the character data from <player>.dat and put it into the player attributes in level.dat? I've tried using NBTExplorer, but it forces me to change everything one at a time—and that would take far too long.