I have alopecia, my hair has fallen.
So I was wondering if I could do the mashah while doing wahdu, if I ready have a valid wadhu, while wearing the wig? Similar to washing feet and wearing socks.
It would be a valid Wudhu if you cannot pick it up (you cannot remove your wig). Hence you can Mash on it.
( in accordance with the ruling of Grand Ayatollah Hadawi Tehrani as a Marja’ al Taqlid of Shia)
But I recommend you to read the site blow which consists of more detailed info. Regarding the mentioned issue. (Of course I could not find it in English), Anyhow, I hope at least you could find your matter in one of these three languages)
http://islamquest.net/tr/archive/question/fa21348 (in Turkish)