ANSWER:

He doesn’t have to add anything as he prayed one single rakah with his imam and that was sufficient.

If the imam prayed 3 rakah and he only caught one, he must stand up after salam of the imam and compete the remaining two. 

Sheikh if a person misses the two rakah of witr and joins in the last one rakah where imam does dua, how/what is he supposed to pray after that one rakah. Like the imam prays 2+1 in witr and the person joins in the single rakah.