There is no making up for the missed days of supererogatory Sawm even if a person missed them willingly. But it is better for the Muslim to continue doing good deeds according to the Saying of the Prophet (peace be upon him):
The deed liked most by Allah is one to which the doer adheres constantly even if it is small.
So you do not have to make up or expiate for your missed supererogatory Sawm. It should be noted that a person gets rewarded even if he fails, due to disease, inability, traveling, or the like, to do good deeds that he used to do as in the Hadith:
When a slave of Allah suffers from illness or sets on a journey, he is credited with the equal of whatever good works he used to do when he was healthy or at home.
Al-Bukhari in his Sahih.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.