There should be no problems. Just add the server to the Pub as a node before you install and have the security password ready. Also confirm that your licensing on the pub allows for that additional node.
Please rate if this helped.
Edit: I didn't read the entire problem. You can only have 3 servers in a CM group. I thought you were asking in a cluster.
If you have more than 3 call processing servers you simply structure your groups to provide redundancy/load balances accordingly. I have one 10 node cluster with 8 call processing servers (8 is max) where I did 4 groups as following:
Sub1, Sub2, Sub4
Sub3, Sub4, Sub2
Sub5, Sub6, Sub8
Sub7, Sub8, Sub6
This way you accomplish 1:1 redundancy where the even CMs are backups, and they are also secondary backups to other groups.
Not sure what you mean, I have 8 servers wich is the max distributed between 4 groups. You can create more groups if you want your deisgn to be different, but you can only have 3 CMs in a single group, CM will not let you add more.
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