What is the ruling on adding "And you, O Fawz of the repenters, seek forgiveness from Allah" after the Imam's statement: "I say this and I seek forgiveness from Allah Almighty for myself and for you" in the first sermon, and is it permissible for the worshippers to seek forgiveness at this point?

Seeking forgiveness (Istighfar) is one of the most sublime acts of worship. There is no harm in concluding the sermon with it occasionally, and it has become a common practice among many preachers, and many scholars have deemed it desirable, provided that it is not done consistently so that people do not mistakenly believe it to be from the Sunnah (definition). There is no objection for the congregant to seek forgiveness or supplicate with whatever they wish between the two sermons, because this time is one of the times when supplications are answered.