The best means of integrating svn externals with git-svn that I’ve seen is this script, which clones your externals into a .git_externals/ directory and creates the symlinks and exclude files you need. I find this a simple and direct solution. YMMV.
Here is an older overview of other options for dealing with svn externals with git-svn. To me they look a little over-complicated and liable to break under subsequent Git use.