If I understood your question correctly ( How do I import contacts directly into a segment without creating any contacts) I would think: because mautic creates AND updates contacts when importing, you could import the contacts directly to the segment and it would essentially create no new contacts but update the existing to be included in the aforementioned segment. Export from there I suppose.
If you are asking how to create segments using variables present in the database but not selectable on the segment filter screen Id ask for an example of what variables you are referring to.