Se o meu google-fu estiver correto, infelizmente parece você não pode fazer isso .
Parafraseando a postagem do TechNet:
You can't install the Windows 8 KMS host Product Key for Windows on Windows Server 2008 R2. You can only install the Windows 8 KMS host Product Key on a Windows client operating system (i.e., Windows 7 or Windows 8).
If your KMS host is a server operating system (Windows Server 2008 R2, Windows Server 2012) then you will have to apply the Windows Server 2012 KMS host Product Key. The Windows 8 KMS host Product Key is insufficient.
This is similar to way that Windows 7 / Windows Server 2008 R2 KMS Host Product Keys work. You need to install the "server products" KMS host Product Key when your KMS host is a server operating system or you need to install the "client products" KMS host Product Key when your KMS host is a client operating system.
Acho que a sua melhor aposta é designar um host do Windows 8 como um host KMS e ativar suas máquinas contra ele.