git subtree merging with a SVN remote?

Hello,

I want to use "subtree merging" to get stuff from an external SVN repo into my 
project.

Is it possible to do this directly using the external SVN server or should I set 
up a GIT mirror of this SVN repo?

Is this "subtree merging" a pure local thing or do I push some information about 
this to the central GIT server?

Yours

Manuel