They are using a technique called motor imagery, which looks for small changes in synchrony in the sensory motor rhythms (SMR). SMR is currently only capable of reliably detecting left hand, right hand, and combined foot imagined movement. When they say "raising a hand" they are not finding a pattern of activity that relates to that gesture, they are simply detecting if a left vs right vs both motor action was imagined. As such, you cannot, unfortunately, simply think up another gesture to add.