Subtree with submodule inside?

Is this even possible? The .gitmodule file has to be at the root of the repository, AFAIK. So if the subtree is inherently not at the root, how does it manage its own submodules?

Basically I have a common library that also keeps a submodule of third party dependencies (binaries). Each super project that depends on this common library (each super project has its own repository) will add it as a subtree. So what happens to the submodule with this setup?

Thanks.