You cannot do this with the past fasting, however, you can combine the intention for future fasting on Mondays and Thursdays and other voluntary fasts.

Recently this year I have alhamdulillah started to fast most of my mondays and thursdays. Ramadhan is coming up, and I missed ten days of fasting last year. Can I use the fasting I did on mondays and thursdays to replace those missed days even though my niat was just to do the sunnah fasting (my intention wasn’t to replace those missed fasting days) ?