Yes, but then all of their sins would have been forgiven at that moment.
The point you raise would lead us to ask if Jesus was only praying for those few who may have been converted or for all those who put Him to death.
If we broaden this consideration to include Stephen, we only know of Saul (who became Paul) being converted from among his killers; so was Stephens prayer only applicable to Saul/Paul or was he praying for all those putting him to death?
None of us really know who of the people that Jesus or Stephen prayed for in the instances discussed except for Saul became saved or not, so why do so many assume they did not? Surely nobody assumes that the only people that were saved are mentioned in the scripture and those not saved are not.