No.
A mod manager is not needed. There is no chance of NWN modules "tripping" over one another the way they do in TESCS.
Unlike Oblivion, NWN/NWN2 modules are self contained. They are not "pasted" into the main module. Each NWN module contains all the information needed to run independently. When needed a module will automatically call up any necessary hacks and tlk files.
All you need to do is put the .mod file in your modules directory and make sure you have any necessary hak pak(s) installed . As long as this is done you will be able to select modules from the start menu.
The only potential conflict that might occur is with hak paks. All hak paks must have unique names (something the building community is very careful about) so they won't conflict with each other, but this isn't something you as a player need to worry about.
Post edited May 25, 2013 by urknighterrant