Should a worshipper interrupt his obligatory prayer, or should he intend it as a supererogatory prayer in order to join the congregational prayer that began during his prayer?

It is better to change it to a supererogatory prayer and then pray with those who are entering in order to attain the virtue of congregational prayer. If you cut it off and pray with them, there is no harm, because cutting it off is for a legitimate Shar'i benefit that redounds to the prayer itself.