Poses and Shape Keys

This is my first attempt at building and rigging a character from scratch, thanks to several on-line videos.
i now need to create some shape keys for the mouth to use for lipsyncing. Is there a way that I can shape the mouth using the armature and then convert this pose into a Shape Key. I guess what I’m asking is how to convert a pose from a pose library into a shape key.
Thanks for any help

I’ve just seen that there is a button to do this in 2.9. Is there a corresponding button in 2.83?