Hi - I don't think this is possible in the current version of the probe. Reason being is the probe is only designed to look at the process cpu/memory usage and has no visibility of the system totals in order to work out the %
What I have done in the past, is run a LUA script upon alert arrival which queries the CDM probe on which the alert was generated and calculates the percentage. At this point it would either amend the alert or close the alert depending on threshold.
This is a work around if you are desperate for this functionality.
It would be worth creating an 'idea' for this additional probe functionality if one doesn't already exist.