Shapekeys Extended
Where do I find the addon panel?
It will be in the same location as the normal shapekey panel, just at the bottom of the list. You can click + drag the top right of the panel to position it back up to the top to make it easier to get to next time.
What blender versions does this support?
I have personally tested 3.6 up to 4.2. Versions before that will have parts that are broken or fully broken.