simply add the activity to the list returned by getSelectedActivities(). Just be aware that you need to add an ActivityRef and not just an Activity to the list. An ActivityRef consists of the activity itself, the row on which it is placed, and the layer where it is shown.
there is no utility method for doing this, yet (please file a ticket). You will need to call scrollTo() on the tree table view and then request a specific time on the timeline (Timeline.showTime()).