So far as I know, this is not possible. But I read some stuff about GIT subtree and submodules, which should do the "trick". But how do I accomplish this!
I found this blog post https://medium.com/sroze/managing-monolithic-repositories-with-composers-path-repository-c28af031746d, but cant get it work for me.
Can anybody help please?
In general it is not necessary to have sub-repositories in a composer based installation of TYPO3. Each extension is handled with it's own repository independent of others.
Without composer you could use the mechanism of sub repositories to build an installation from the base repository which includes all extensions with their original repository.
I'm not sure whether composer handles sub repositories in a correct way as sub repositories need further git commands to fetch/ update the sub repositories.
Or you could get conflicts when the main repository requests another version of an extension than composer.
And if you want to use composer only to manage your main repository and handle the sub repositories by yourself you miss the advantage of composer.