Is there a way to enable premium users to read and write private text that basic users cannot access? For example premium user writes a post with the first paragraph as normal content, then the second paragraph as private content. The first paragraph would then be viewed by anybody but the second paragraph would only be seen by premium users.
Can this be done with XenForo software or would an add-on be needed?
With that addon members have to be invited to view the whole thread. The OP is wanting a way to limit certain parts to regular members and the whole post to another premium type usergroup so that addon you linked won't work for their needs/wants.
@geeksquad maybe you can talk to @SneakyDave about extending or modifying his thread markenator addon to accomplish what you're wanting cause as far as I know it's the closest thing there is to what you want.