NOTE! Developers are recommended to work with 1.9 at this time. Only maintenance and security changes are being made on the 1.7 branch.
Please consider sharing your modifications with the community and helping integrate them into the standard code!
Subscribe to the freeside-devel mailing list. Send your changes as unified diffs (diff -u). If possible, send changes against current CVS HEAD and also any branch they should be applied to (i.e. FREESIDE_1_7_BRANCH). However, if all you have is diffs against a release, well, probably best to send what you have rather than waiting.
Also note that the license requires that you provide source code if you provide hosted access to a modified Freeside instance.
Signup and self-service
You can of course customize the signup and self-service templates as well as plug into the self-service API from Perl, PHP or any other language via XML-RPC. See the FS::SelfService API.
The backend code is structured to make it easy to add "plugins" of various types. 1.7 allows you to easily add new price plans and exports. 1.9 extends this and additionally allows event plugins (conditions and actions) and new kinds of service tables.
Additional developer information
Schema changes - Quick documentation on how to make schema changes