Is there a way to create a new material for part of an existing body part.

M4's chest is actually his chest up to his lower neck.  I'm applying displacements to create the veiny muscle look, but when that's under clothes there's a lot of pokethrough only in the render.

• Removing displacement from the chest area solves the render poke through, but then the lower part of his neck has no veins while the upper part has veins.

• Inflating the clothing (shirt) to solve poke through makes the shirt far too large and puffy looking.

If the chest area beneath the shirt could be selected and assigned a separate material, then that area could be done without displacement so there wouldn't be pokethrough in the render.

But if I create a new body part and assign a new material to it, with grouping tool, then seams appear.  It looks like both the original material is applied to chest, and the new material to the area of the chest selected for the new group.

In short, can existing body parts and material groups be separated into pieces, so different materials can be applied to different parts of the chest, for example?