There appear to be tool bar buttons for many features, including "Clear Playing Now". There does not appear to be a selector for "Add to Playing Now", other than to select items, rt-clk and select the "Add to Palying Now" option.
While this certainly works, it can be tricky since it is quite easy to either select the wrong option of slip and deselect some or all of the previously selected tracks.
It would be very nice to have a button on the tool bar to add, just as one can have for clear.
Regards,
Milt