Sorry I can't answer your question officially...
...but I'm pretty sure that it'd be illegal to convert any copyrighted module without permission. They probably won't prosecute you if you don't distribute it. They'd probably even leave you alone if you did. They might start getting mad if you charged for it. And I'm sure they'd sue your pants off if it caused them to lose lots of money.
Incidentally, I've already converted an entire module using the beta toolset. My very own "Hello, World!" was none other than...The Orc and the Pie!
I've considered trying to get a group together to work on converting RttToEE (but then I realized that would end up being a full-time job).
At any rate, if you wanted to do some old modules, I bet you could convince whomever holds the copyright to them to let you use the material. If they're not selling any more copies of the modules, and could reap some royalties from sales of your products, they've basically got nothing to lose.
Of course, you'd also have to consider the licensing agreement on the NWN tools (whatever that may be).