Weight paint against current pose? I want to add some new mesh object to my posed avatar.

But by default weight paint is against rest pose. So when paint in current pose, the new mesh would "fly" away.

Is it possible to treat as if the current pose is the rest pose, without actually setting the pose as rest pose?
It's absolutely common and default to weight paint over a  posed character. If I understand your question, try using this buttons while editing the mesh, which enable the armature action during edit mode of the mesh.