Got no responses... but, I figured this out myself.

In CCP, add *one* MSC device only. Set primary RF ID to >=10. This allows all 12 ports to be available to the MX-980. Then, set secondary RF ID matching legacy remotes to < 10. This allows the legacy remotes to be recognized by the MSC.

BUT!!!! All IR devices referenced by the legacy remotes must now be placed on ports 7 - 12 of the MSC via sleeved emitters. And, when referencing devices in legacy remote programming, map ports 1 - 6 on remote to 7 - 12 in the MSC. For example, Port 1 in the MX-850 is actually Port 7 on the MSC, etc.