Hello,
you can use below method for pass value from SO Line to Stock Picking.

   Below is a Example from source of Odoo V10.
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    @api.multi
     def _prepare_order_line_procurement(self, group_id):
        vals = super(SaleOrderLine, self)._prepare_order_line_procurement(group_id=group_id)
        for line in self.filtered("order_id.requested_date"):
            date_planned = fields.Datetime.from_string(line.order_id.requested_date) - timedelta(days=line.order_id.company_id.security_lead)
            vals.update({
                'date_planned': fields.Datetime.to_string(date_planned),
            })
        return vals

I hope it will help you.
Thanks