What do you mean by material slots? If it is present in the Godot editor, then in theory it should be possible since the Godot editor runs using Godot. That said, depending on what it is, it might not be easy.
Do you mean you want to access materials within a script and be able to populate those materials through the Godot editor? If so, you just need to create an exported material. Something like this should work (untested):
export (Material) var material_slot_one
# or you can specify what type of material you want
export (SpatialMaterial) var material_slot_two