Here's my answer:
Currently there's no feature to import data into SectorMaker. The .nsec files that it creates are in a proprietary format.
I'll add your suggestion to my list of things to do for the next version.
Joshua Bell has a nice page on the format I currently use for export:
He also has a nice page on the possible formats I could use for importing data:
On that page the T5 formats are nice because they include the extended data like Importance, Economic Extension, etc. So I wouldn't have to randomly generate this data on import. For the legacy formats I would have to randomly generate the extended data.
If you search Traveller Map for a sector and click on the data link at the lower left, you get a text file of T5 data. So this is a good candidate for the kind of file I could import.
Let me know of any sources of data you know about that would good candidates for import and I will look at them.
So I'll be considering adding this functionality as well as other things in the next version.