Safari Developer Program site is to place to go to submit your extensions. As we’ve mentioned
before, developers can apply to this program for free (no $99-per-year fee like for the App Store), though they must register for a digital certificate from Apple to distribute with their extensions. The certificate serves as a failsafe Apple can in theory use to disable an extension in the eventuality that it violates Apple’s terms and conditions.
Though Apple is now accepting developer submissions for the Safari Extensions gallery, there is no new word on when said gallery will be available for Safari users to browse. We also don’t know how much Apple will be curating the gallery, which could continue to leave room for third parties to collect extensions that don’t make it into Apple’s showcase.