First create your joint name it something appropriate so you can find it in the list of other joints.
Select the joint hold shift and select your mesh.
Go to skin>edit smooth skin>add influence SETUP
Now turn the default weighting to "0" at the bottom. Then turn "weight locking on".This will attach your joint to the new mesh without messing with you current weighting.
Now select your mesh and go to:
skin>edit smooth skin>Paint Skin weights tool
Find your joint in the list of bones that appear in the paint skin weights settings. Below the list is a "toggle Hold Weights on selected" button. Press this to remove the hold on the new joint you created.
And you're done!
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