You can signup for an account at http://svn.modxcms.com/jira/ and create ticket for the appropriate version. You can then attach a patch to the ticket and/or create a review in crucible (http://svn.modxcms.com/crucible/) where you can upload the patch for us to review in the context of the existing code in SVN. This will be the most effective and efficient way to contribute such things.
Consistent contributors who get patches accepted by the team will likely be invited to enjoy commit privileges eventually.
Alternatively, you can post the patch in the forums and someone on the team might have time to grab it, do the above, and get it committed, but this will likely be a slower process, since most of our existing contributors are volunteering their time.
Cheers,
-Jason