Hi,

For adding new values to an existing selection field in Odoo, you can do it by using the selection_add , see the below example,

alias_contact = fields.Selection(selection_add=[('employees', 'Authenticated Employees')])

Original field definition:

alias_contact = fields.Selection([
    ('everyone', 'Everyone'),
    ('partners', 'Authenticated Partners'),
    ('followers', 'Followers only')], default='everyone',
    string='Alias Contact Security', required=True,
    help="Policy to post a message on the document using the mailgateway.\n"
         "- everyone: everyone can post\n"
         "- partners: only authenticated partners\n"
         "- followers: only followers of the related document or members of following channels\n")

Thanks