Mesh as a shape key

I have two meshes, a cube and a sphere for example, is it possible to add the sphere for the cube as a shape key? In order to be able to manipulate between these two shapes using a slider.

Have a subdivided cube
Add the base shape
Add a shepekey
Make the cube a sphere with Shift + Alt + S / 1
Change the shapekey slider to move from one shape to the other

A different process would be needed for more complex shapes

Yea, it would be easy to transform one shape into another, but what if I can’t do that? Could you tell more about complex shapes? I thought maybe there are any special functions, because actually I have two human bodies so it’s impossible to transform one of the bodies manually.

If the objects are the same but in a different shape you can transfer the shape of one object to another as a shape key

Thanks for the link. “Join as Shapes” works fine for me. It seems I was not persistent enough, because the solution is very simple. But this function requires equal numbers of vertices on both meshes, it might be a problem for some.
The other way is to add a Shrinkwrap modifier and Apply as Shape Key, it works fine on a cube and a sphere, but something goes wrong with complex shapes.
Thanks again, good luck.

Exactly why you need to be specific in your questions.
If you ask a vague question expect a vague answer. If you have a specific example you must always supply a demo blend file for review