Does it have to be part of the report header or do you just want to pop a text box in the report every time you make a report?
Adding a text box to a report is fairly easy but it isn't permanent. Changing the report header is much more complicated, but you can change it to be anything you want.
Is your part number populating from the Part Number input in the header of your program, or are you using variables/tracefields with a custom header in DataPage?
If your part number comes from the header of your program and you are using DataPageStats.exe, then you can modify the XML file that DP+ imports. It will create 2 seperate entries in DP+. If StatsOn/Off, then I don't know.
If you are using tracefields, you can modify them through the data entry tab in DP+ (I am not at my computer, and i forget exactly what it is called). The CAD tab reports though, as far as I know, are not modifiable once populated.