I made a Manuel Bastioni model. I wanna add a downloaded animation to it. But the animation armature's rest pose is a T-pose while my model is an A-pose. so I can't add that armature to my model. What should I do? If I change the rest pose to match my model, the animation keyframes of the armature get messed up because they are relative to the original rest pose.
In principle you have to bring your mesh into the position that it matches the given armature. One way would be to create your own armature, position your mesh so it alligns with the downloaded armature and press "Apply" in the Modifier tab.
But probably the Sculpt Brush would be a better solution for you: https://developer.blender.org/D5647
Remember to make a backup of your file before you start modifying the mesh.