It is Mustahab (desirable) to use Siwak before Salah, not during it, as it entails some kind of movement. Also, it was not reported to have been used this way. The Hadith states:
Were it not for fear that it might be hard for my Ummah (nation based on one creed), I would have ordered them to use Siwak at the time of every Salah.
That is, before commencing to pray.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.