The auto-overlay package can easily be extended by adding new overlay classes1. The next sections document the functions and interfaces provided by the auto-overlays package for this purpose.
Often, a new class is a minor modification of one of the standard classes. For example, it may work exactly like one of the standard classes, but in addition call some function whenever an overlay is created or destroyed. In this case, it is far better to build the new class on top of the existing class, using functions from the class-specific Elisp files, rather than starting from scratch. See Standard Parse and Suicide Functions.
[1] Or rather, it is easy to integrate new overlay classes into the package. Whether writing a new overlay class is easy or not depends on what you're trying to do, and how good your coding skills are ;-)