Hacking a WPMUDev Plugin for Fitness Tracking

I would like to hack an existing plugin to track site users' fitness activities and achievements.

Maybe Events+, because it has mapping, user specificity, etc. As long as I can add custom fields, it could work.

I know that there's a Fitness Tracker app for WP already, but it's quite trivial. My thought is that by hacking an existing extension, I can benefit as the underlying code improves.

I'd appreciate any tips.