The two cards I am concerning.

Flayer of the Hatebound: Whenever Flayer of the Hatebound or another creature enters the battlefield from your graveyard, that creature deals damage equal to its power to any target.

Evernight Shade: Pay a black: Evernight Shade gets +1/+1 until end of turn

If I control a Flayer of the Hatebound, and I cast an Evernight Shade from the graveyard (most likely with Chainer or Dread Return) can I dump mana into the Evernight Shade to beef it up before Flayer of the Hatebound causes it to deal damage?

I don't know how creature abilities stack on top of their own ETB.

The two cards I am concerned with:

Flayer of the Hatebound:

Undying (When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.)

Whenever Flayer of the Hatebound or another creature enters the battlefield from your graveyard, that creature deals damage equal to its power to any target.

Evernight Shade:

{B}: Evernight Shade gets +1/+1 until end of turn.

If I control a Flayer of the Hatebound, and I cast an Evernight Shade from the graveyard (most likely with Chainer or Dread Return) can I dump mana into the Evernight Shade to beef it up before Flayer of the Hatebound causes it to deal damage?

I don't know how creature abilities stack on top of their own ETB.