I’m afraid, SVN Mirror add-on has limited support of SVN Externals and it is not able to translate SVN externals to Git submodules or subtrees. The only thing it’s able to do with externals is to translate them into a special .gitsvnextmodules file that would contain some text information about externals, and that’s pretty much all.
As a workaround, the repositories connected as externals can be translated to Git separately and the connected to the main Git repository as submodules/subtrees. Or they can even be connected to the main Git repository right on the server with our new add-on Git X-Modules:
This add-on does exactly that – it adds one Git repository content to another as a subdirectory, for example, which may allow repeating the SVN repository structure on Bitbucket.