How can someone who does not wish to pray Witr with the Imam intercede when the Imam prays Witr as three continuous rak'ahs, or five rak'ahs or more?

Most scholars permit exceeding two rak'ahs in voluntary prayers by night and day. They interpret the hadith: "The night prayer is two by two" as indicating preferability and guidance towards what is lighter.

Some scholars, such as Malik and some Hanbalis, hold that it is not permissible to exceed two rak'ahs in voluntary night prayers.

If the Imam prays Witr with three continuous rak'ahs and the worshipper wishes not to leave until the Imam leaves, then he should make the Imam's Witr prayer even by praying a fourth rak'ah.

As for the other view (prohibiting the increase), if the worshipper wishes not to leave until the Imam leaves, then he should pray Witr with the Imam and then pray as much as he wishes afterwards, without repeating Witr.