Everything is done, you have your repository published somewhere, and you can add the repository from your QGIS. You’re wondering if you can put your repository as a default repository that will be shown to every other QGIS users without them needing to add manually the repository. Well, yes you can do that.

To make your repository listed as default repository, please fork this Github project QGIS official resource repository, update the directory.csv file and make a pull request to the upstream project. If you are not comfortable doing this or you don’t have Github account, please email me (akbargumbira(at)gmail.com) and tell me the detail of your repository.