I would not suggest any manual method because it seems to be quite complicated. NSF to PST Converter is the quick solution I would say on your situation.

Visit here: http://www.nsf.pstfile.org/

Re: How to convert NSF to PST free?

You can try the IMAP method to convert from NSF to PST.

First backup your data on NSF platform. Then create Office 365 mailboxes for users.

Then port the NSF data to Office 365 mailboxes through enabled IMAP ports.

Thanks.

Regards.