Change root directory for materials drop-down

So for most 3D objects, in the details panel, there is an option to choose the static mesh and material to set to it.

I was wondering if it was possible to change the root directory that this list searches from. Currently, the root directory is, by default, set to /Game. I want to set it to a folder within /Game. Can this be done with blueprint structs? If not, what about C++ or blueprints in general?