Assuming we're talking about NWN2, there used to be a file that allowed this. I looked it up, but it seems it's no longer applicable.

http://nwvault.ign.com/View.php?view=nwn2ui.Detail&id=61

However, according to the last comment: "However, since the chance of any updates beyond 1.23 is nil there is no further need for this as long as you are playing the Windows version. Just open the toolset and select the Campaign Editor from the Plugins menu. In the editor select the campaign you want to alter on the left and then in the right pane change AllowUnrestrictedLevelUp to True and then click on "Save Campaign"."

I haven't tried it (it's been years since I played the OC), but that seems straightforward enough for you to give it a try. Hopefully, it works.