If you want to add actual powerups, there is an entity - info_powerup_spawn, but it only works if you are in powerup mode: tf_powerup_mode 1.
If you just want to add powerup effects - you can use a prop_dynamic with a model you need (for example, Haste powerup model, can't remember the model name, but I think it can be found by searching for "mannpower" or "powerup" in the model broswer) and trigger_add_tf_player_condition on top of it
https://developer.valvesoftware.com/wiki/Trigger_add_tf_player_condition