I don't have all 5 finger bones in my rig. What to do?

Let's say you want to create a missing pinky.
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Duplicate the ring finger joint chain, and move it a little bit to the position where the pinky finger should be.
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Rename the bones to something which makes more sense, like pinky_1.L, etc.
And that's it.
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Double-check if the first Pinky joint has the same parent joint as the first Ring joint.

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