Keeping voluntary fasts (e.g. – the six fasts of Shawwāl or ‘Āshūrā or fasting Mondays and Thursdays etc) before making up the fasts missed during Ramadān is neither permissible nor valid.
Even if one swore an oath to fast for a day, they would have to do it after having made up for the obligatory fasts from Ramadān.
This ruling applies even if the individual has enough time to make up the obligatory fasts before the following Ramadān.
This is the Maddhab.
Q. This applies to sisters who have fasts to make up from Ramadān due to menstruation? Yes.
Q. Can the intention of making up obligatory fasts be combined with the intention of a recommended fast? No.
[Refer to Kashāf al-Qinā’]