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If you're using CBB for XOOPS 2.2.3, there's a setting called "Allow requiring reply to read a post" under general preferences. I think it does what you're looking for.
I don't think that it's possible to restrict access via post count like you want. From what I can tell, all forum permissions are handled through groups now. You might be able to make a particular group of people who match the criteria you want, and then populate it as you go. But that kind of sounds like a pain.