Long story made short, we were playing Dead or Drowning and if Tomble lived 1 attack we would have won the game. When he got attacked he was next to a knocked out hero whom had a defense pool of 2 gray dice. After the attack dice were rolled and Tomble's original defense die were rolled it gave him just enough damage to knock him out. However, if Tomble had an additional 2 gray defense die there is a very good chance that he would have survived.

So does Tomble get to add an adjacent knocked out heroes defense pool?

You played that correctly. A knocked-out hero is not on the map and thus cannot be adjacent to anything including Tomble.