I think it's more like you can't add new types of activities, but you can add more of the same activity. You could have a hundred different Activity_Waiting entries as long as the number on the end is different. So I can't add Activity_KickMonsterInTheFace since that doesn't already exist, but I can add Activity_Waiting100 if I wanted.
Activities are a little flexible. In the past, I've added some "new" ones to the Rock Raider in order to support custom vehicles. You will not find the following activities in the base game, but they work. Go into the Pilot.ae file and look at the activities for vehicles (what the raider does when driving vehicles), and you'll see something similar.
;ON R3F1
Activity_R3F1 StandST
Activity_StandR3F1 StandST
;ON Bulldozer
Activity_Bulldozer StandBull
Activity_StandBulldozer StandBull
Activity_ClearBulldozer ShovelBull