One of the big problems with mesh hair is control it in an animation.
So, with that in mind, here is how you can 'rig' a wig and then parent it to the body armature. This allows you to build a model with as many wigs as you like.
I'll try to keep this as simple as possible. The first and most important step is to duplicate the head bone, and separate it out as a new object. Be very careful not to change its rotation or location. Then, using shift + alt + ctrl + c set the origin to center of mass of the hair mesh. Use ctrl + p for automatic weighting. Then parent the wig rid to the body rig.
After that, just follow the settings in the screenshots:

