I've done this in another software (FTM), but I'm sure you can do it in reunion as well. I did it for both person events and marriage events, as I was tracking who was showing up as batismal sponsors as well as witnesses to weddings. I made two custom events for each:
1. Godparents of [or you could use Sponsors of]
2. Godparents are
1. Witness of ---
2. Witnessed by ---
To do the latter, go into the marriage record. Click on the Events tab. Click on "Add events" at which point a menu of Marriage Events options comes up. Below the list, you'll see "Event Preferences..." Click on it. The next screen enables you to create a new event by clicking "Add Event" on THIS screen. Call it what you want and give it the preferences you want. If you want it to be a default event that always shows up as an option, then copy it over to the "Default Family Events" column. Then close the window. Your new event is now in the list of available events to select for the family/marriage.
You can do the same thing for a person's baptism.
p.s. and of course you're using Reunion; this is, after all, the Reunion forum! ;-)