How can i use shapekeys on shapekeys more comfortable?

I have a character that has different poses and each poses has it’s own constant shapekey.
On that, I want to add shapekeys to improve the look of the animation.
The current solution, is to animate between shapekeys that are almost the same.
To make a normal shapekeys workflow, I will have to apply the constant shapekey to the mesh, but then I’m losing the other poses that character has.
Is there a way to make shapekeys on constant shapekey without all that mess?

Sounds like you need driver bones for the shape keys.