Is the addition of the Field in SAP GUI Transaction - CRMD_ORDER is valid one. As you have mentioned you are using SAP CRM 7.0 - SAP has already recommended using the new WebUI for all the Business Functionality and SAP GUI only for administrative purpose. Also, you should take care of whether SAP will support SAP GUI CRMD_ORDER or not.
You can make the changes in the CRM BOL and GENIL Layer using the Easy Enhancement Work Bench. That will add the fields but for the UI you have to make use of the Enhancement Set and Configure the changes in it. And you can add many Custom Fields based on your requirement.
You can have a look at the following link for [Easy Enhancement Work Bench|http://help.sap.com/saphelp_crm50/helpdata/en/9f/a19c921f0911d6b1d500508b6b8b11/frameset.htm]
Hope this helps.