VCenter UM 5.1, upgrade Esxi 4 Host to 5.0

We are an MSP and have a central Vcenter 5.1 server for all of our clients who have a mix of ESX 4.0 to ESXi 5.1 licenses and hosts. I want to upgrade ESXi 4.1 hosts to ESXi 5.0, I am entitled to 5.1 but can only go to 5.0 due to vendor requirements (Cisco). I can import a 5.1 iso to UM but can't import a 5.0 ISO. Is there a package that is available that am I missing to allow me to do this?

Thanks!