The permissions sections are confusing, and I haven't tried this, but, I think it'd go
something like this:
1) Create a new group, something like "Full Members"
2) Set "Post Count" to 200
3) Update, then click "Site" to edit permissions
4) "Copy Permissions from Users" to carry over current settings
5) (There may be a simple SQL query for this part, but, you want to assign users with 200+ posts to the "Full Members" group...) Perhaps Member Management > Advanced Search Options... click on "Users" and "Member Post Count: (greater than 199)"; Submit
6) Action: Change Groups, "All Members in Search"
7) Add to "Full Members"
8) Back in Group Management, you'd want to Edit Site Permissions for "Users" to set "Total # of private topics allowed" to zero to disable it.
That
should add your current 200+ post-count users to the "Full Members" group, with PM permissions, and disable PMs for "Users" (with less than 200 posts). I'm not all that familiar with this permissions system, so I'm not exactly sure how to assign
new members to the "Full Members" group automatically as they reach 200 -- I'm sure someone else will chime in here -- but that will hopefully set you into the right direction at least...
Basically, you're assigning these people to a specific group -- and most importantly, it's not the Moderators group, but a slightly modified version of the
Users group.