How to use subtrees when importing SVN repository with "vendor" branches?

Hi all,

using git-svn, I've been converting our old SVN repositories to git with great success, but I don't know how to deal with our biggest and most important one. The SVN repository structure is like this:

	branches/
	tags/
	trunk/
		ExtLibs/
			libpng/
			zlib/
			...
		some_dir_a/
		some_dir_b/
	vendor/
		libpng/
		zlib/
		...

The key problem is that we used to use the SVN "vendor branches" strategy: When a new version of libpng or zlib or ... is released, we update the vendor/ directory appropriately (essentially: delete the old files of the library, extract the tarball of the new release, commit). Then we "SVN merged" the vendor/ directory into trunk/ExtLibs/. This way, we were able to preserve our occasional customizations to the libraries in ExtLibs/ while updating to new vendor releases.

Using something like
	git svn init "svn://.../project_root" --trunk "trunk" git_test_project
it would be easy to fetch the projects trunk (never mind the branches and tags, we never 
used them much and are happy to ignore them). This properly converts the whole trunk to 
git, including subdirectory trunk/ExtLibs/, but with no notion of vendor/.

In contrast, using something like
	git svn init "svn://.../project_root" --trunk "trunk/ExtLibs" git_test_ExtLibs
	git config --add svn-remote.svn.fetch "vendor:refs/remotes/vendor"
	git svn fetch
yields a new clean "ExtLibs-only" git repository with the two branches trunk and vendor, 
and proper merge history (helped with grafting, where necessary), but is so far entirely 
unrelated to the main project repository.

Well... it seems like subtrees and subtree merging is a good way to proceed, but even though I've read everything I could find about the subject, I don't quite see how I can employ it in the case above, where ExtLibs/ isn't newly created, but history already exists.

Could you please give me an idea how I properly merge ExtLibs as a subdirectory into the main repository, including the history? I can add the ExtLibs repository (created as shown above) as a remote reference to the main repository, yielding a multi-root repository, but what then? Or should I rather convert only vendor/ and use that as a remote for merging? (Can grafting put to work in the subtrees context?)

I'd be very grateful for your help!

Best regards, Carsten