Can I use Sam's response ability if I play The Hammer-stroke when the enemies are already engaged with me?

No. An enemy is said to engage a player when it moves into that player's area from another game location.

Page 7 Online Rules Reference:

Engaged :
When an enemy enters a player’s play area from another game location, that enemy has engaged that player, and that player has engaged that enemy.

Thank you for super fast answer 🙂

That's an interesting text in this situation. Perhaps it would be better worded to say "each non-immune not engaged with you engages you." I believe that is the intent on the card.