Is there any way to convert NSF into an IT file?
Midis are way too limited.
ITs are also very limited. If an instrument would contain more than one duty cycle, you would need more than one "instrument" at the IT side.
Well, something like a MOD or something. A tracker format that supports custom sounds.
If I play a .nsf that plays a square wave and changes its duty cycle (in CPU $4000) each frame, the duty cycle will change 60 times a second no matter how I set the pitch registers ($4002-$4003). If I create a sample of this duty cycle, the duty cycle will change much faster for high notes than for low notes.
You can make a tracked music file that sounds enough like an NSF by recording each track of the NSF to a separate WAVE file and then imitating each sound in each channel of the file. If you can make a cover version of the NSF using NerdTracker II, that would be a first step.