Because of the wording for the hammer ("after resolutions"), the extra action token that gets applied on a roll of 10-12 is not used when determining how much damage would be applied by Iceman's Glacial Wall.

For example, if you place the Barrier token adjacent to two opposing characters, one character with no action tokens and one character with a single action token, and if you hit both characters with an attack roll of 10-12, the following happens:

The character that originally had 0 action tokens is given an action token from the attack and is dealt 1 damage. It then gets a second action token due to the hammer.

The character that originally had 1 action token is given a second action token from the attack and is dealt 2 damage. This character now has 2 action tokens so the hammer cannot apply a third action token.

Keep in mind though what the rules say about pushing damage. If the character(s) getting a second action token don't have Willpower, then that's an additional point of unavoidable damage that is dealt.