Hi, Bob.
You're right that this is a limitation of the current set of built-in suffixes. Most users may have been using the prefix "Dr." for MDs and DDSs. In addition, the current version does not 'learn' from new user entries for default lists. This is something that has been asked for by other users and is a feature that will likely be a part of a future major release.
Thank you for your feedback and sorry for the inconvenience,
Support Team