Do the following,

1. Create a new custom module if you dont have already
2. Inherit crm.lead 
3. Add the following code

class CrmLead(models.Model):
   _inherit = "crm.lead" ​   ​    ​   ​

priority = fields.Selection(selection_add=[('4', 'Fourth'), ('5', 'Fifth')])

4. in selection_add for Fourth and Fifth replace whatever text you want.

5. Install /  upgrade custom module. It it add new priority start.

A good article to read

Extend Selection Field