How to add metallic and smoothness to a frag shader?

I have a dissolve shader that’s implemented as a frag that I’d like to also have support metallic and smoothness. Anyone know how to do this in a frag? I tried adding a 2nd pass as a surf and setting it there, but that completely overwrites everything that was done in the frag. Is there a way to provide the output of the frag to the input of the surf?

I’m trying to add some Smoothness and Metallic to my custom shader in order to have dark texture for day/night cycle mybkexperience