Is it permissible to fast days other than Mondays and Thursdays to make up for missed fasts?

Your question is unclear. If you doubt the validity of your fast, this doubt is not to be taken into consideration, because doubt concerning an act of worship after its completion has no effect. If you doubt the obligation to make up specific days, then you are not required to make them up. Fasting on days other than Monday and Thursday is a good deed for which the doer is rewarded. It is not a condition for a make-up fast (qada') or a voluntary fast (tatawwu') to be on these two days. Many scholars consider it disliked to fast only on Friday or Saturday voluntarily.