Some features cannot be made perfect, also not through lua.
As i explained in the other request-topic about IT import, the pitch envelope has a different scaling range than Impulse Tracker itself has (Renoise has 24, IT has 32). So to get that perfect for adaption this would mean:
You have to check the whole song for which tracks use that instrument and which notes, than figure out which scale is used and hopefully not the full scale is used. The latter means you can shift the pitch envelope settings for that instrument and adjust the notes in the tracks accordingly.
If both the full positive and negative range is being used, you can’t correct anything here.
This is only one feature difference explanation.