When I choose a power granted by an entity, is it adding the power to the click or replacing it?
Example: Zoom has super senses. If he is possessed by Black Hand, can I add invulnerability to his ss or is it replaced and Zoom now has invulnerability?

Powers or abilities granted by an external game effect, such as entity possession, only replaces powers or abilities on the character if any game effect specifically says so. And there is no such statement in entity possession so the new power it grants is in addition to any powers the character can allready use/possesses.

Just note that if a character has more than one power that reduces and/or ignores damage dealt then only one of those may be used per each instance of damage dealt. Which power or ability is used is decided by the character's controlling player.

That said, super senses doesn't reduce or ignore damage, it evades an attack. In this instance, when an attack is made against Zoom, you get to roll for super senses, then if hit reduce by two. You can't stack reducers (toughness/invuln/imperv) together or with impervious, but you can avoid damage in many ways. For instance, you can use shape change (can't be target of an attack) super senses (evade attack), and impervious (reduce damage to zero or by two) and theoretically have three separate rolls to to try and avoid damage.

Thank you. Zoom has traited shape change, so that will be helpful.