Answered by Shaykh Faraz Rabbani
Question: Can I Combine My Intention for a Missed Ramadan Fast and An Optional Sunnah Fast (Such as Shawwal, Muharram, Mondays, etc.)?
Answer: In the Name of Allah, Most Gracious, Most Merciful
Walaikum assalam wa rahmatullah,
Yes, it is valid to make the intention to make up missed Ramadan fasts and to make a secondary ‘sunna’ intention–such as fasting the six days of Shawwal, or the sunna of fasting Mondays and Thursdays.
And Allah alone gives success.