It doesn’t look like that plugin allows you to do such things, unless they update it to allow you. I am using my own API calls via the API connector and passing in “traits” as an object when doing POST to https://api.segment.io/v1/identify. You can read about this in their documentation:
If you haven’t already, I’d suggest getting familiar with crafting API calls using POSTMAN (download it from the Chrome browser store), then copying your setup into Bubble’s API connector so that you can add custom traits when you are identifying your users (or anything else you’d like to do). There are a number of posts in this forum for learning about API Connector.