macmiller:
You can open the object for editing by placing it, selecting it, then use the "Open Object..." command.
In the 3D Script of this object you will see near the top where the various leaf and bark materials have been defined, in the Parameter script are the commands for the leaf and bark material pop ups, and in the Master Script is the reassignment of the chosen material name to the material name in the 3D Script (mat1 and mat2).
One approach would be to create 2 new parameters for mat1 and mat2, and set their type to Material so they are assigned just like the Pot and Earth materials are in this object. You would also want to comment out the Leaf and Bark commands in the various scripts to disable them.
HTH
David