Is it possible to add nested submodules in the root repo in git?

I want to make submodule in my repo:

Root /
\--- /A (submodule)

But also I want to nest another submodule, also in root:

Root /
\--- /A (submodule)
 \-- /A/B (another submodule)

And I can’t add B as submodule to A, since A is a third-party repo.

The real-life example.

llvm + clang trees: I want to include both repos, but clang must be checked-out inside llvm tree.

Git Baby is a git and github fan, let's start git clone.