When a trustworthy narrator narrates an addition that was reported from a group of transmitters, his addition shall be accepted according to most scholars whether the addition was in wordings or in the meaning because as it is permissible to narrate a Hadith from his way only, he may narrate an addition from his way only.
If it is said: It is impossible to narrate an addition from his way only while other narrators are listening to the narration.
We say: The confirmation of other narrators is more important, so if a narrator reported an addition while others did not negate the addition, perhaps the Prophet (peace be upon him) said it in two sessions and in that session only a single narrator was present or the Prophet (peace be upon him) reported the same Hadith in a second session and mentioned the addition in one of the two sessions in which a single narrator was present.
It is also probable that the narrator who narrated the short-form narration came in the middle of the session and did not hear the narration from the beginning or narrators who attended the session forgot the addition except one of them. Or something came up that interrupted the narration and some narrators were distracted except one of them. So, in all these cases the one who listened carefully to the narration may get the entire narration, whereas some listeners may be distracted to hear the entire narration or left before the narration was complete.
If one of the previous cases took place, there is no way to belie the narration of a trustworthy narrator.
The general rule is that it is not imaginable that a Muslim would narrate something from the Prophet (peace be upon him) and add to it something he did not say, particularly when he heard or was reported to him the Hadith of the Prophet (peace be upon him): "He who ascribes lies to me must take his place in Hell-Fire."