The only field you can customize is the "Severity" field. The label of this field can be changed, and you can add new values to the drop down list.
To edit the label of the field go to
CONFIGURATION | CUSTOMIZATION | click "Severity" | Change Value | Click Update
The name of the field will be changed.
To add new values go to
CUSTOMIZATION | LIST OF VALUES | Click "Add Values"
You can add new values, and they will then appear on your drop down list.