[Q] How to add a device to a repo

Hi all, i have been busting my head and still cant find like a proper tuto to do this.
If a repo does not have a phone/tablet added into it, how can i go ahead and add that device to a repo, so it can be synced?

example: add android_device_xxxxxx to the repo list in the repo github

What i will need to change or where.

I would really appreciate your pointing finger to the right way for me to get this.

Thank you.