I would just import them directly like a vendor branch.

http://marc.info/?l=git&m=119495815518898&w=2

It works like this: (I just did this test)

#inside of your repo

git remote add vendor $vendor.git

git fetch vendor #grab their code

git merge vendor/master #imports their code into your repo

git mv vendorlib lib #move it where you want

git fetch vendor #grab the next version

git merge vendor/master #git remembers your mv from before

*update

of course since this is git there is yet-another-command called git read-tree to do a lot of this.

It's all described here: http://git-scm.com/book/en/v1/Git-Tools-Subtree-Merging