Is there way to achieve this without breaking support for projects that already use git submodule ?

I'm not sure we actually need a rad command for this. So that request is a bit of a red herring :)

The submodule incompatability that's been showing up is due to radicle-surf choking when it comes across one while generating the file-system entries based on a Git tree. A patch to radicle-surf has been merged, and a new version has been published. We're waiting on merging of a radicle-httpd change.

That is to say, I don't think there is any extra work in Radicle to support submodules. The regular git tooling should work as is. One can use git submodule add and point it to the URL of the repository. In fact, I believe you could use the https URL of a seed, and I think you could use rad:// but that would have to be tested (someone could easily add a test under examples/git and see :)).