You could put something like Quickverb on the tracks in Multitrack just for the recording session with the vocalist, and then take it off again afterwards. No damage done, no sweat.
Or you could do a mixdown of the whole session and then export just that stereo track to a new session in Multitrack. Then add any reverb you like and use it for recording the singer. Then export the singer's tracks back to the original multitrack session.