All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Purification is not a condition for any of the Hajj rituals expect in Tawaaf around the Ka‘bah. When ‘Aa’ishah experienced menstruation, the Prophet, sallallaahu ‘alayhi wa sallam, said to her: "Do what the pilgrims do except Tawaaf around the Ka‘bah." [Al-Bukhari and Muslim] Therefore, ablution is not a precondition for throwing the pebbles. It is only recommended.
Allaah Knows best.