Hi,

You can use the selection_add attribute to add new values into the existing selection field. You should inherit the sale.order model and add the new state to the existing selection field.

eg:_inherit = 'sale.order'
state = fields.Selection(selection_add=[('c', 'C'), ('d', 'D')])

Here's a video for your reference:https://www.youtube.com/watch?v=wsf0y_2AVP8&ab_channel=CybrosysTechnologies

To add button and to call function when clicked, you can refer this video:https://www.youtube.com/watch?v=IPJwnFNb6RQ

Hope it helps