What version of JabRef and what version of LibreOffice is this?
I think what you want to do is doable with JabRef
This specifically is doable with customizing entry types. Check the “required” and “optional” flags. Customize entry types | JabRef offers a little bit of explanation. You also can check for consistency and move data from fields to other fields via the Automatic field editor.
I suppose your custom format of jstyle files is not something JabRef or Mendely supports importing. JabRef natively follows the Bibtex (further info: bibtex/tamethebeast and biblatex standards. It can import and export from and to various other standards, but if you import a custom one, it obviously will lead to weird outcomes.
Maybe it helps, if you simply “open” the file though, instead of “importing”, because importing will try to map fields, whereas opening doesn’t as far as I am aware. I would check out Import | JabRef too.