link folder to external repository [ svn propedit svn:externals vendor ]

export SVN_EDITOR="mate -w"
svn propedit svn:externals vendor

and input like the following to the popped editor

rails http://dev.rubyonrails.org/svn/rails/tags/rel_1-2-3
radiant http://dev.radiantcms.org/svn/radiant/tags/rel_0-6-2/radiant/

this would make the folder like this.
vendor
  |--rails
  |--radiant