Is it permissible to be absolved from a vow to pray all prayers in the mosque, with the addition of two rak'ahs as expiation for not attending the mosque for, for example, the dawn prayer?

The vow of praying all prayers in the mosque must be fulfilled. If a prayer is missed in the mosque, then either what was vowed must be performed, or an expiation for breaking an oath (kaffarat yamin) must be paid. The expiation for breaking an oath is feeding ten poor people or clothing them, or fasting for three days for those who are unable to do that. One cannot be absolved from a vow except in the case of inability, and in that case, the expiation takes its place.