Hi Alpesh,
Sadly there is no way to do this in Odoo right now, its a limitation of the current framework. You'll either need to use selection_add to add the element at the end of the selection:
state = fields.Selection(selection_add=[('to_approve', 'To approve')])
or recreate the whole field definition from scratch so you can add the selection value at the place you'd like:
state = fields.Selection(selection=[
('draft', 'Draft'),
('to_approve', 'To approve'),
('posted', 'Posted'),
('cancel', 'Cancelled')
], string='Status', required=True, readonly=True, copy=False, tracking=True,default='draft')
Regards,
Yenthe