Is it permissible to make up for missed Ramadan fasts from previous years during the first ten days of Dhu al-Hijjah, with the intention of making them up rather than fasting these ten days voluntarily?

Completing one's repentance entails making up for the missed days of Ramadan. You are not required to pay any expiation (fidyah) as long as you were ignorant of its ruling. It is permissible to make up for the missed days during the first ten days of Dhul-Hijjah, with the hope that Allah Almighty will combine for you the reward of the virtuous time and the supererogatory act.