HR Based workouts are supported yes. Mixed mode workouts are also supported (Power + HR based)
You can see more here.
To do that, you need to have the outside ride loaded and synced to the app via DropBox
The “pair workout” feature will pair the workouts planned (for the day on intervals.icu) if you have multiple planned workouts that did not get paired automatically.
When you pair within the BreakAway app, what happens is, it will pair the planned workout with the actual workout, remove the planned workout from the calendar and merge it with the actual workout. Same as what intervals.icu does when you do the drag and drop.