(My first answer to this appears to have been (accidentally) deleted).
You might be able to replace your use of SVN externals with Git submodules, which would reference whole repositories though (and not just specific files/subpaths).
You could have a separate directory for the submodules and then only symlink to specific files from where you need them, for example.
There is
SubGit, which would allow you to seamlessly migrate from SVN to Git (by using both in parallel).
But that would make sense only, if you could use a remote Git repository for synchronization (Github) and is probably not worth the effort, if you intend to switch completely anyway.
See also
https://help.github.com/articles/importing-from-subversion .
Cheers,
Daniel.