I wouldn't have a clue. Also remember that rpt files are binary files, not text. you can interact through code to do many things, but I don't know if it is possible to add a new field onto an existing report.
At the very least, it would need to be field that is already defined in the dataset that the report was designed with. If the field doesn't exist in the dataset, I don't believe that is possible.
I haven't done any manipulation like that as it is simpler for me to design a report with fields that move from side to side and turn the visibility on and off...but you can do that from inside the designer.
There are probably methods that you interact with, but I don't know how robust they are. The only ones that I have seen are concerned with setting of datasets or parameters, but I haven't used them.