HoMM VII Haven Units – Haven faction units from the seventh installment of Heroes of Might and Magic.

Well, the model isn't in a T-pose, and the crossbow from the seventh game is simply longer than the one from the sixth, so I don't know.

The crossbow, whether in a T-pose or not, is the same. It doesn't "bend." If the crossbow from the seventh game is larger... then you can simply increase the size of the crossbow from the sixth game to match. What's the problem?
I'm saying all this because it's a simpler solution than editing a bunch of animations.
Or, as an alternative, you could take the crossbow from the model in the seventh game and use it for the model in the sixth game.