I doubt that it will work. Due to the way that Crystal works, it will have already have read the data, and will not read again.
As suggested before, create a stored proc and in it, you can create the field and populate it with the data that is desired. A view would work as well.
Last suggestion, and for something like this, I don't like, but you could create a subreport to get the value that you need....Crystal will do a data read again for a subreport. Reason I don't like it, it will incur lots of hits to the DB for something that could have been gotten in another manner much more effectively (via view or stored proc).