CRM filed mapping

Hi,

My requirement is to ceate order in an opportunity entity.  So I'm using below code for generating the salesorder.

 GenerateSalesOrderFromOpportunityRequest req = new GenerateSalesOrderFromOpportunityRequest();
            req.OpportunityId = targetEntity.Id;
            req.ColumnSet = new ColumnSet(true);
            GenerateSalesOrderFromOpportunityResponse resp = (GenerateSalesOrderFromOpportunityResponse)_service.Execute(req);
            Entity SalesOrder = (Entity)resp.Entity;
            return SalesOrder.Id;

Suppose if there are  100 fileds that are mapped from an opportunity to order .But how do i mention the request so that a particular filed not to map between those entities while creating an order from opportunity.

Thanks