Here's the scoop. The MX-3000 is only capable of ID 01-0F but that is not significant. You do not have to set an ID greater than 0F to get IR ports 7-12. You can set the MSC400 primary to 01 and the secondary to 02.

In the MX-3000 software go under RF setup and set the Default Receiver ID to 01. Then click 'add' and give it a name such as "secondary" and set that one to 02.

Now to properly route to ports 7-12 use the following
Port7 = Secondary Receiver Port1
Port8 = Secondary Receiver Port2
Port9 = Secondary Receiver Port3
Port10 = Secondary Receiver Port4
Port11 = Secondary Receiver Port5
Port12 = Secondary Receiver Port6

There is text at the bottom of the RF Setup window in the MX-3000 software that explains this as well.

Hope this helps.