Open Data Editor and go to Behaviors.
There right click anywhere and click on "Add Object".
After that you'll have a screen pop out and there add your name and id of the behavior, and change the type to veterancy and click ok.
You should now have a behavior ready to set up, but you now need to set up every level exp requierments.
To do that search for the line that reads "Veterancy Levels +" and there in the first field add every level and experience requierment.
If you cand see "Veterancy Levels +" try clicking on the orange squares on the top.
After you are done with this, simply go to your unit and search for "Behaviors +" and add the hero behavior"