.

#1 In the motion panel with the biped selected, expand the layers drop down.
Click the create new layers button.
Bookend your frame range with 2 keyframes on the problem bones and adjust the rotation of the hand.
Check your animation and ensure the change is behaving correctly.
Press the collapse layers button to bake the animation down.

Cheers :)

ps. The help menu is your friend. When in doubt, F1!!

#2 If you still have problem mocap around.
I would like to work with it in Motion builder.
I have not used it yet since I purchased it.

:)