You can use (material as any).blackandwhite = ... or (material as MyMaterialWithBW).blackandwhite if you create a MyMaterialWithBW class that subclasses Material and which is adding a proper blackandwhite property. I think the latter way is better because later on you can access directly material.blackandwhite instead of doing (material as any).blackandwhite each time you want to get access to the plugin.
Note you can also retrieve a plugin from a material by doing material.pluginManager.getPlugin(pluginName), so technically you could simply do:
BABYLON.RegisterMaterialPlugin("BlackAndWhite", (material) => {
new BlackAndWhitePluginMaterial(material);
return null;
});