There is a method named _prepare_order_picking in sale.order model in sale_stock module that generates and returns a dictionary to creates delivery order.
You have to override that method and modify the dictionary as per your need.

class sale_order(osv.Model):
    _inherit = 'sale.order'

    def _prepare_order_picking(self, cr, uid, order, context=None):
        vals = super(sale_order, self)._prepare_order_picking(cr, uid, order, context=context)
        vals.update({'field_name': your_value})
        return vals

I guess this will solve your question.