Some people say just use a NPT endmilll with the right taper built in and mill just like a straight end mill. We have used this method for 20 years. Some people say you have to follow the taper outward as you mill the thread.
I have a macro NPT milling routine that does that. You will have to have Macros active on your control. If you PM me your E-Mail I can send it to you. I also have a Macro to do straight threads you can have. It probably depends on the degree of error you can stand and how much pressure is on the joint.
You can do a search and find a thread from several years ago that had a lengthy discussion about both methods.
Edit added link