you can expand the 'priority' fields value in the task object.

currently the field look like :

priority = fields.Selection([('0','Normal'), ('1','High') ], default='0', index=True)

now, if you want to add more option for selection, do something like in the task object:

priority = fields.Selection([('0','Low'), ('1','Normal'), ('2','High') ], default='0', index=True)

note: if you doing something like, you need to change the code related to this on everywhere.