Subtree merging of sub-directories between repositories?

Is it possible to use the subtree merge strategy to import just one directory from a repository into a directory of a different repository? The subtree merge example describes bringing in an entire repository as a directory, but I'm not sure how to adapt that to just import part of the source repository. The repositories are completely different, but repository A has a couple directories I want to import into repository B (along with the history that affected those directories). I may also make changes on either side and it would be nice to be able to re-merge in either direction.

I was thinking I might need something like the git-subtree script, but maybe this can be done with standard git commands?