Import part of a SVN repository into an existing Git repo

I have an existing SVN repo and I would like to import a part of it, with history into an existing Git repo. Could anyone give me a high-level overview of the commands needed to do this?

Summary:

• I want to move some subdirectories in an existing SVN repository (but not the rest of the repo)
• I have an existing Git repo that I want to import those directories into
• I want to preserve the history of just the subdirectories I moved (but not anything else)

I have a feeling this involves something with git filter-branch and some strange merges. Any help would be appreciated.