It turns out that we do not recommend or support making edits to core files. What I can suggest instead is contact one of the developers listed here and they may be able to contract with you to build this feature into a small plugin:
If you are a developer or there’s a developer on your team and while working on extending the checkin functionality, and it’s found there’s a need for some new hooks to make this happen, we welcome pull requests via our project on Github:
The support post ‘CheckIn’ is closed to new replies.
Have a question about this support post? Create a new support post in our support forums and include a link to this existing support post so we can help you.
Support forum for Event Espresso 3 and Event Espresso 4.