If you have multiple KMS servers in your domain, or want to add an other KMS server from outside the domain, just ad an SRV entry in your DNS.
| Name | Setting |
|---|---|
| Name | _vlmcs._tcp |
| Type | SRV |
| Priority | 0 |
| Weight | 0 |
| Port | 1688 |
| Hostname | FQDN of KMS Host |
With the priority you can add a preferred server. If you choose a higher number for the kms record, this record comes after the record with “0”.
Source: http://technet.microsoft.com/en-us/library/ff793405.aspx