Is expiation obligatory in addition to making up the days during which one masturbated in the daytime in past Ramadans, and how does one make up those days without knowing their number, and is it permissible to combine the intentions of making up (fasts) and observing a Sunnah (fast) when fasting them?

Intentionally discharging semen during the daytime in Ramadan (definition) invalidates the fast. It only requires making up the fast (qadaa') according to the majority of scholars, and expiation (kaffara) is not obligatory except for breaking the fast through sexual intercourse. You are required to make up the days you invalidated by masturbation, along with a fidyah (definition) (expiation) of feeding one poor person for each day you delayed making it up past the following Ramadan, unless you were ignorant of the prohibition of delay. Your combining the intention of making up the fast with the intention of safeguarding your chastity does not affect this.