Straight from the docs...

Note: You can limit the content for a DynamicPopupFolder by using pattern matching. If the DynamicPopupFolder contains another folder, that folder must match your pattern in order to be displayed.

Example: *Popup "My Documents" !PopupFolder:"C:\My Docuemnts\"*.doc

Multiple directories can be placed in the same PopupDynamicFolder by using the pipe character, "|", to separate them.

Example: *Popup "C Drive" !PopupDynamicFolder:$Programs$|"C:\"