Tashahhud is an obligatory part of Salah which must be performed exactly as the Prophet (SAWA) and Ahlul Bayt (AS) taught us to perform according to the authentic Prophetic Hadeeth (Perform Salah as you saw me performing Salah) (Sallo Kama Ra'aitomooni Osalli)صَلّوا كما رأيتموني أُصَلّي.
Imam Jafar Al-Sadiq (AS) taught his followers all details of Salah including Tashahhud. ( Amaali Al-Shaikh Al-Sadouq , page 498 narrating from Hammaad Bin Easa).
Tashahhud must have ALLAHUMMA SALLI 'ALA MUHAMMAD WA AALI MUHAMMAD which mentions Imam Ali (AS) and all the Infallible Ahlul Bayt (AS).
We are not allowed to add on Tashahhud more than what Ahlul Bayt (AS) taught us.
We can mention the name of Imam AALI (AS) in the form of Du'a within our Salah after Tashahhud, like saying : ALLAHUMMA SALLI 'ALA AMEERIL MO'MINEEN. Du'a is always good within our Salah and not only in Qunoot.
By this Du'a, we will be mentioning the holy name of Ameerul Mo'mineen (AS) and not adding on the Tashahhud which was been taught to us by Ahlul Bayt (AS).