I have a lot of LibraOffice Writer documents with fillable fields and would like to convert them to O365 Word. Is this possible and if so how?
Probably not.
Microsoft Office 365 can open odt files directly. Not everything is supported, see Differences between the OpenDocument Text (.odt) format and the Word (.docx) format - Microsoft Support
If you have access to 365 then you would be best placed to test.
Export to pdf form always works