I do not think this is possible with VSE.
The only way you can add custom detections is with the "Potentially Unwanted Programs" and setting a User Defined Detection in the policy. As you are looking for what appears to be a "*.PST" this is not going to be a possible option as it will not allow wildcards. This stops someone adding an entry for *.EXE which would most likely kill the box by deleting all .EXE's until the OS fell over.
As a solution, you could write a batch file to search the local disks for *.PST's ( excluding as required ) and create a text file with the path/filenames, machine names, username etc. You could then copy this to a netowork share with the associated <computername.log> filename.
If you wanted to go further, within the same batch file you could use that file list to then run a loop / delete command. This could then be initiated from a login script or even packaged up with EEDK and deployed as a task via ePO.