There are twenty (20) fields in the Opportunity Manager that can be relabeled only. You can not add fields to the OPMGR table.
Any relabeling of the fields would be global and apply to all opportunity records. To change the labels go into the Opportunity Manager and click on the Customize button. Click on the Labels Tab and change the fields as desired.
Another good feature to consider is to create templates for standardized opportunity categories. Once established you can select a template to prefill fields and save time with data entry. For new users and administrators the Opportunity Wizard helps guide you through the opportunity creation.
Many users decide to uncheck the wizard then they struggle to get an opportunity entered properly. I tried in vain to get my sales staffs to use the opportunity manager over the years without success. We ended up going back to just scheduling and completing forecasted sales. In addition, the OPMGR table is difficult to report on because parent/child records exist in the same table.
You can add up to 5 user-defined fields to the Opportunity Manager. Click the 'Customize' button in the Opportunity Manager. Then, under the fields tab, click the 'New' button. It is recommended that you backup the database before performing the rebuild. These fields are added to the [OPMGR] table
It is also important to understand that the fields you add will also be added to the Projects Manager as well, as these use the same underlying tables in the database. As a rule of thumb, if you plan to use both modules then you will want to use these five fields for items that will carry over from Opportunity to Project. For additional field data beyond that, I recommend using the Details/Links tab in Opportunity Manager. The 'Item' field acts as the field label and the 'Reference' field is used for the field data which can have it's own lookup list based on the selection in the 'Item' field. You can then add these Detail records to an Opportunity Template and they will be added automatically whenever you select that template. The data for the Details/Links tab resides in the [OPMGRFLD] table.
I agree with the above posts AND, when you want to report on this opportunity data with some agility, you'll want to use MasterMine for GoldMine. The built-in opportunity reports aren't going to cut it with these custom fields. http://www.mastermine.net