I'm not sure if such an option exists in MCNP but I know of an alternative. There are some modules in PyNE ( https://pyne.io/index.html ) that you can use to create a custom material libraries, mix those materials as you please, export the materials to mcnp input format.

Material: https://pyne.io/usersguide/material.html

mcnp: https://pyne.io/pyapi/mcnp.html#module-pyne.mcnp

you will need to install PyNE on your computer and other dependencies. https://pyne.io/install/index.html

To use PyNE for example:

run creatematlib.py to create the material library where all the materials that you want to mix will exist.

run mixmat.py to mix the materials created from the previous script either by volume or weight fraction (you will need to make changes to the mixing function) and export them to mcnp input text.

This is the easiest way that comes to mind and I personally have used this tool many times before to mix materials and prepare mcnp input.

Hopefully that helps answer your question.