Hi,
Is it possible, during mesh animation (we use md5) to change, for example, 1 or more joints position?

For example, I have a walking human, but I want to change the rotation of his arms according mouse cursor.

Could you please help

See the following thread and the ones linked from it and see if you can find any relevant information. http://away3d.com/forum/viewthread/2676/

There is no feature for this exact purpose in Away3D, but the skeletal animation system can be used to make it happen, by modifying the joint poses manually.