I'm not sure I understand. You already have all those trappings under your skills.
But, to clarify, it doesn't work for a 'single exchange', it works for a single 'challenge or action'.
So, if you are shooting someone with a gun, a Guns replacement item would work for a single attack.
If you are talking to someone, a Rapport replacement item would work for a single check.
So, if you are in a social conflict and you end up rolling rapport 6 times, you might use your item once. That result might represent several minutes of conversation.
If you are in a social situation where the GM calls for a single roll to resolve the conflict/scene, you can use your item and that result might represent 15minutes, or a half-hour, or a whole evening, depending on the length of the scene or conflict.