Hi,

You can add a new item to the action button by following code in the sale.order object.

<record id="sale_order_confirm_view" model="ir.ui.view">
    <field name="name">sale.order.confirm.form</field>
    <field name="model">sale.order.confirm</field>
    <field name="arch" type="xml">
        <form string="Confirm Sale">
            <p class="oe_grey">
                Only records of state Quotation and Quotation Sent will be Confirmed.
                Other will get skipped
            </p>
            <footer>
                <button string="Confirm" name="sale_confirm" type="object" default_focus="1" class="btn-primary"/>
                <button string="Cancel" class="btn-default" special="cancel"/>
            </footer>
        </form>
    </field>
</record>

<act_window id="action_sale_confirm"
    multi="True"
    key2="client_action_multi" name="Confirm Quotation"
    view_id="sale_order_confirm_view"
    res_model="sale.order.confirm" src_model="sale.order"
    view_mode="form" target="new" view_type="form" />

Then Once you click the confirm button in the wizard the python function in the transient model will get called.

class SaleOrderConfirm(models.TransientModel):
    _name = 'sale.order.confirm'
    _description = "Wizard - Sale Order Confirm/Cancel"

    @api.multi
    def sale_confirm(self):
        """filter the records of the state 'draft' and 'sent', and will confirm this and others will be skipped"""
        quotations = self._context.get('active_ids')
        
        #here in quotations you will get the id of the selected records
        
        quotations_ids = self.env['sale.order'].browse(quotations).\
            filtered(lambda x: x.state == 'draft' or x.state == "sent")
        for quotation in quotations_ids:
            quotation.action_confirm()

Inside the function you will get the selected records from the active_ids in the context.

quotations = self._context.get('active_ids')

Code is from this module in the store, please look at for reference: Sale,Purchase Mass Confirm and Cancel

Thanks