Du`a' and Isti`adhah by the Ma'mum whose Imam does not say them
At times, the Imam (the one who leads congregational Prayer) may recite Ayahs (Qur'anic verses) that mention Jannah (Paradise) or Hell-fire in Jahri Salah (Prayer recited out loud), is it permissible for the Ma'mum (a person being led by an Imam in Prayer) to ask Allah to grant them Jannah and protect them from Hell-fire
if the Imam does not do so? Also, is it permissible for the Ma'mum to send blessings on the Prophet (peace be upon him) when the Imam recites Ayahs which mention the Prophet (peace be upon him)?
To be on the safe side, a Muslim should do so only while offering a supererogatory Salah (Prayer). This is because it is reported that the Prophet (peace be upon him) used only to do so while performing Tahajjud (optional late night Prayer) and Allah (Glorified and Exalted be He) says:
Indeed in the Messenger of Allâh (Muhammad صلى الله عليه وسلم) you have a good example to follow.
Moreover, the Prophet (peace be upon him) said:
"Pray as you have seen me praying."
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.