Hi,

You can of course submit a merge proposal directly to the official addons project, following the procedure described in the documentation. The contribution guidelines also highlight the main rules for accepting contributions in the official branches, and determining the appropriate target series.

Basically if you're fixing a bug you can make a MP towards the relevant stable series (your commits must link to the bug report as explained in the guidelines). If you're adding features or improving existing features you should submit your work on the trunk (development) branch, or possibly publish a separate module providing that feature.

PS: Alternatively you can submit your patch to the OCB branches, and roughly the same rules will apply, but the target audience is different. Note that having your patch accepted in the OCB branch does not guarantee that it will be accepted in the official branches. There is currently no automatic process for passing changes from OCB to official, so it's always recommended to submit your patches to the official branch in any case.