Default statistics cannot be edited but they can be duplicated and modified if you need - we would not want someone accidentally causing their VPIP stat to display the wrong information but if they choose to create a modified version having the existing version as a starting point might make things easier for them.
Custom statistics can be created in the registered version of PT3, they are not available in the trial.
Custom stats are available for download
here. Any statistic in the Player Statistics section should be available for HUD use.