Menu: Edit -> Add Case Filter…
Drag and drop: see below
Adds one or more Case filters to the selected filter or subfilter. The Case Selection Dialog will be opened. Select the Case(s) you want to add and press OK.
As the new Case filters are added side by side under the selected filter or subfilter they work as a logical OR expression.
Using drag and drop:
You also can drag a Case from the Case filters folder of the filterset pane and drop it over a filter or subfilter to add it there (as subfilter).
See also: Building Filters