Shape Keys In Blender

Can I do this for say trying to make a shape key on clothing for when say someone activates shape keys on female models for like "Small Booba" and "Big Booba". Sadly I tried to use the information here to create an easy "Even bigger booba" shape key without success because it seems to be capped at 0.000 out of max 1.000 on the shape keys (aka when I move the slider nothing chances on the keys).

Is it possible to use this method for rigging poses

cool