There are 3 remotes, Primary, Secondary and Tertiary.
Secondary is a fork of Primary with more additions. Tertiary is a fork of Secondary with even more additions.
Secondary is kept in synch with Primary by adding an upstream to a local copy of Secondary and then fetching+merging from Primary and pushing back to remote Secondary. No problem here.
Now, I need a local copy of Tertiary (or any other local copy of forked Secondary) to be aware about Secondary's upstream to be able to fetch+merge from Primary without adding Primary manually.
Also, I need to know how can I 'chain' these upstreams, that every next fork (in depth) would know about all parent upstreams and would be able to synch with all or any of them.
So the main question is: "How to save upstream to remote repo so anyone who will fork it later will be aware about upstream?".