Ref. No. 914/41-39
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
If a woman fasted in the month of shawwal to makes up her missed fast days of Ramazan (obligatory fasts) with the intention of Qaza, she has fulfilled her religious obligation and also obtained the reward of fasting 6 days of shawwal. But the woman must intend only to make up what she has missed from Ramadan.
Nevertheless, if she is in a good health, it better to fast the 6 days of Shawwal (Voluntary fast) separately.
أَقُولُ فِي فَتْحِ الْقَدِيرِ صَامَ فِي يَوْمِ عَرَفَةَ مِثْلَ قَضَاءٍ أَوْ نَذْرٍ أَوْ كَفَّارَةٍ وَنَوَى مَعَهُ الصَّوْمَ عَنْ يَوْمِ عَرَفَةَ أَفْتَى بَعْضُهُمْ بِالصِّحَّةِ وَالْحُصُولِ عَنْهُمَا (انْتَهَى) (غمزعیون البصائر 1/150)
And Allah knows best
Darul Uloom waqf Deoband