Using Existing Materials as Material Layer

Hi, New to UE.
Hit a wall creating material blends. This is for product viz where I want labels and stickers on objects or material mixes but using existing Materials in the library.
I have managed to do it building from scratch with material layer but cant find a way to get an output from, eg. a complex Automotiive paint, that I can use as a Layer.

The layer slots only accept ‘Layers’ so how to take a material or material instance and make it dragable into one of those slots?

Thanks!