Many people on the forums seem to be confused about how to add a role to an existing Exchange Server when a Rollup (RU) is applied.
Let's say we have an Exchange 2010 Client Access Server with RU1 applied. We now want to add the Hub Transport Role to this Server. Do I have to uninstall the RU, add my new role and then re-apply the RU?
Well the answer is no you don't have to. Be content in the fact you can add extra roles without having to perform further Administrative tasks. When Exchange setup runs, it looks into the MSI to figure out what files and other things should be on the box for the “Add Role Scenario”. and if the MSI Service detects that a Rollup is installed, the MSI Service will combine both the MSI and the Rollup into a single entity and then install what is needed.
MVP - Exchange Server