How do I sync contacts who are not members in Wordpress?
There are two solutions:

1) You can set the Organization Membership to "trickle down" via a relationships. This is hard to change after the fact. You will need to update the Membership type via the database and then manually update all the Memberships. 

2) You can create an advanced search pulling all the Organization Memberships and set the Display Results As: Related Contacts. Make that into a Smart Group. Use that Smart Group or most likely multiple Smart Groups into a Group Role Sync via your plugin.