Question: It is claimed, and only Allaah knows the truth, that if a woman is duty-bound to make up for a number of Ramadaan missed fasts, it is permissible for her, while embarking upon the recommended fast of the six days of Shawwaal, to say: “O Allaah, I intend both obligatory and supererogatory fasts”. What is the Sharee`ah (Islaamic law) ruling on this?
Answer: Making up for some Ramadaan days only discharges the obligation of missed Ramadaan fasts; thus, it is impermissible to combine another kind of fast with it through one intention. However, it is Mustahab (highly encouraged) and an abundantly deserving reward to fast the supererogatory six days of Shawwaal after making up for the missed fasts of Ramadaan. Anyway, the above-quoted saying has no basis in Sharee`ah… read more here.