Turning on and off Always Deform

The method works for editing the mesh, editing bone, even adding bones.

I only have one arm in the mesh and need to:
mirror copy the bones and
rename and include them in the existing skin.

In Advanced Parameters, turn OFF Always Deform.

 

Rename and add the bones to Skin ...

Put Always Deform back ON ...

Mirror mode ... paste blue to green bones
[the completed side color to the new side color]

 

... then, paste green to blue verts ...

... and test animate ... the left arm is behaving exactly like the right arm.