Hi,
It should work. You can add additional SRM pair (with custom ID).
Existing installation can be leaved as is.
As per
http://kb.vmware.com/kb/1014640
"Installing SRM with a custom extension does not preclude you from creating an additional installation, extending the same instance of vCenter that uses the default extension. Similarly, installing SRM with the default extension does not preclude you from creating an additional installation, extending the same instance of vCenter that uses a custom extension. You can run SRM with custom extensions only; the default extension is not a requirement."
Michael.