Is it permissible to perform Umrah and donate its reward to the souls of the martyrs of Syria, knowing that their number is large and there is no kinship or personal acquaintance with them?

If the intention is for him to perform 'Umrah and make the talbiyah for himself, and then dedicate the reward of the 'Umrah to a group of people, there is no harm in that. He will be rewarded for his good deed, and the reward of the 'Umrah will reach those to whom he dedicated it. However, if the intention is for him to make the talbiyah for 'Umrah on behalf of this entire group, then that is not valid.