Great.

Here are the steps:
-- Download matmatic. Download the version that corresponds to your version of Poser (P7).
-- Have a read of the matmatic manual provided with matmatic.
-- Study the shader you want to over-write. It's not that difficult, in most non-matmatic-generated shaders.
-- Create this shader in Python code
-- Study the shader you wish to combine with the shader already on the material zone.
-- Create this shader in code.

Study both of these, then decide whether there is functional duplication or conflicting material setups. Resolve, compile and load.

Now, how easy was that? All you have to do is learn a new skill - Python - which is really not that hard to do. And it will stand you to good stead!

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks