Select the hand object, then shift-select the body object, Ctrl-J to join selected objects. I believe armatures can be joined in the same way. Then you will have to go into edit mode for each the mesh and the armature, and clean things up. For instance, the hand and the body will be one object, but the vertices will not be connected at the wrist, so when the hand is posed in an extreme position a gap maybe visible where the two join. Same thing goes for the armature, yea it’ll be one armature, but the hand bones won’t be connected to the arm bones.
He he… yes there probably is, if you joined the main Armature to the hand armature and the main Mesh to the hand mesh, it’s possible but may come with more problems than you bargained for.
When you animate or pose the final armature it has to work out the space and rotation of each bone that travels thru a hierarchy like this - bone > Parent bone > Chain > Constraint > Armature > Object > World. Now, if you added your Hand-Armature at exactly the same Space and Rotation as the main armature, and you have done no kafuffling with either armature’s axis after that - not the bones but the Armature as the wrist bone will still become the child of the next bone in the chain of the main armature, but the main armature will inherit the axis properties of the Hand-Armature - then you may be okay.