Afaik, those buttons are hardcoded in Blender and you cannot add them to your pie.
But you can add 3 separate buttons to select Vertex/Edge/Face mode (Command tab):
Vertex mode:
C.tool_settings.mesh_select_mode = [True, False, False]
Edge mode:
C.tool_settings.mesh_select_mode = [False, True, False]
Face mode:
C.tool_settings.mesh_select_mode = [False, False, True]