There isn't an easy way to do this presently. There is a related issue
that I was planning to work on soon:
http://code.google.com/p/creoleparser/issues/detail?id=45
If all that's needed is a common class for all external links,
it would be very easy to implement. If I don't get any feedback to the
contrary, I will be adding that support within a day or so.
Stephen