Some alarms contain triggers that are not supported in the vSphere Client and cannot be changed. However, you can still configure the alarm actions, enable or disable the alarm, and change the alarm name. If your environment requires changes to these alarm triggers, create custom alarms by using the vSphere Client or the VMware vSphere APIs.
This might come in handy
VMware vSphere 4 - ESX and vCenter Server
Hope this will help .
Best regards
Your Oscar