Is it permissible to observe voluntary fasting to make up for days missed from Ramadan or to atone for oaths sworn regarding things one did not do?

Supererogatory acts do not substitute for obligatory ones, nor do they take their place. Therefore, whoever has an obligatory fast to perform cannot compensate for it with a supererogatory fast. Similarly, supererogatory prayers do not fulfill the obligation of prescribed prayers. As for expiation for an oath (kaffarat al-yameen), it is done by feeding ten poor people, or clothing them, or freeing a slave. If one is unable to do so, then by fasting three days. This fasting is obligatory, and a supererogatory fast cannot compensate for it.