Are you using a Data Page (a.k.a. Declare Page) to obtain the list of values for the drop down or are you using a Local List in the Property Definition? If using a Local List in the Property Definition you can manually set the order in which you want the values to appear.
I dont think there is any OOTB way to do it.. I recommend to reach out to Pega with a SR (request for information). They can confirm if there is any OOTB way to do it.. if not, for time being, we may have to implement our own custom functions..
Based on request, Pega may come up with a enhancement fix.