Contributions to Botan are definitely welcome. To increase the chances your patch will actually make it into releases in a timely fashion, here are some guidelines for patch submission.
All changes should be sent as unified diffs, created either by diff or monotone. There are instructions available which show how to check out the latest revisions of Botan using Monotone, which is the source control system used by Botan.
Don't send whole source trees, with various changes throughout, as it makes reviewing the patch difficult and can substantially increase the time it takes for your patch to be integrated into a release. Many good contributions were never merged because an unversioned tarball was sent, instead of unified diffs against a specific (recent) version.
Make sure it is obvious which version of Botan your patch was made against.
Patches are best directed to the botan-devel list.
Don't forget to update the file credits.txt with your information, if you want to be listed as a contributor. Also, if your patch is substantial enough to be considered copyrightable, add yourself to license.txt