vendor drop from 3rd party subversion repository

What is the best way to do a vendor drop from a 3rd party Subversion
repository? Currently we use:

% svn export http://3rdparty.com/svn/trunk/3rdparty
[...]
% svn_load_dirs.pl http://example.com/svn/trunk/vendor 3rdparty 3rdparty
[...]

However since both our repository and the 3rd party repository are
Subversion, I wonder if there is a better way?

Specifically, we are trying to do vendor drops from the MediaWiki
repository into our Subversion repository, to manage our local changes:
http://svn.wikimedia.org/svnroot/mediawiki/trunk/phase3/

Thanks and best wishes, Jack