Is it permissible for a congregant to intend to pray two supererogatory rak'ahs (Qiyam al-Layl) behind an Imam who is praying Witr, thinking that the Imam has not finished the supererogatory prayer, or must he change his intention to a Witr rak'ah?

It is permissible for the ma'mum (follower in prayer) to intend two rak'ahs (units of prayer) of qiyam al-layl (night prayer) behind the imam (prayer leader) with whom he prays witr (odd-numbered prayer), and then, after the imam concludes with tasleem (salutation), he adds one rak'ah to make it an even number (shaf'). The difference in intention between the ma'mum and the imam does not cause harm, and performing qiyam al-layl with witr is considered valid by some scholars.