Adding existing models as Blender shape keys

I need to do some morph target animation. I already have the models with the same vertices etc, things like mouth open, eyes shut and so on.

I have been playing with shape keys and can do that fine with a new mesh.

What I don’t see is a way to add meshes/models into from already existing models. So I would start with my ‘neutral’ model and add into it the other models like the mouth open and so on.

I did find the same query in this forum, but the methods have either changed or the links to what I need are broken.

Could someone point me in the right direction please ?

Many thanks.

In the shape keys panel there is a triangle below the + / - button. This gives you more options including ‘Join as Shape’

Join as Shapes Transfer the ‘Current Shape’ from a different object. Select two objects, the Shape is copied to the active object.

Great thanks !