I think you’d need to write a plugin to do that, as you’d need to create your own UserCustomField (slightly different from the ones you can create in the UX currently) to do that and then add it to the serializer, and then display it wherever you want it to show up. If it’s just one field, a place to edit it on the user profile, and then to display it on the user profile (when other users look) and/or the user card, I’m guessing $500-1000.