Pie Menu Editor 1.18.7

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]