You can add ‘custom’ fields in Crystal Reports.
To add a ‘custom’ field to a report, you create a Formula Field and use a formula to define the field’s content.
To create a Formula field, right-click on the Formula Fields entry in the Field Explorer Bar and select the New option. Specify a suitable name for the field and then use the Formula Editor window to specify the formula for the field.
For information on using Formula fields, see the documentation supplied with the Crystal Reports application.