This is possible with the current PRTG Versions. It isn't described in PaeValueLookup.xsd because it is an optional parameter. In XML Schemas, attributes are optional by default. Only required attributes have to be explicitly declared in the xsd.
To make the adjustment/change:
You first need to figure which lookup the channel is using. For this, click on the channel and review the "Lookup"'s name.
Head to "C:\Program Files (x86)\PRTG Network Monitor\lookups" (Or the \custom subfolder) and locate the lookup with that name.
Once you've located the lookup, adjust the undefinedState in the second line of the Lookup:
|<ValueLookup id="ptf.scheduledtask...." desiredValue="0" undefinedState="Warning">|
If the undefinedState="" statemente doesn't exist on the second line, just add it to the end of the line and make sure that you don't violate the XML Syntax.
Once you've performed the required changes, head to Setup > Administrative Tools in PRTG's web-interface and issue a re-load lookups.
More Information about Lookups
Luciano Lingnau [Paessler Support]