Is it permissible to combine intentions when fasting Ramadan; that is, to fast the obligatory Ramadan and another act of worship?

The madhhab of the majority of scholars is that the day in Ramadan is designated for it, so it is not permissible to fast anything else therein. Thus, if a resident, traveler, or sick person intended to fast for expiation (kaffara), a vow (nadhr), a make-up (qada'), or a supererogatory (tatawwu') fast, or simply had a general intention for fasting, their intention and fast are not valid, neither for what they intended nor for Ramadan. Fasting in Ramadan is only permissible with the intention of fasting Ramadan. And whoever does otherwise, the fast of that day is void and they must make it up.