Create a file in a checkout folder, let's say EXTERNALS.txt
In this file set a target folder and a svn source, one per line, similar to this (notice you can do comments with #):

CMFBibliographyAT	https://svn.plone.org/svn/collective/CMFBibliographyAT/trunk
ATExtensions            https://svn.plone.org/svn/archetypes/ATExtensions/trunk
### Third party add-ons ###
ATBiblioList		https://svn.plone.org/svn/collective/ATBiblioList/trunk
ATBiblioStyles		https://svn.plone.org/svn/collective/ATBiblioStyles/trunk

Then set the svn:externals property using this command (notice the dot at the end):

svn propset svn:externals -F EXTERNALS.txt .

This way you can have a visual reference that the current folder gets its content from different sources. You can also see the external sources with

svn propget svn:externals /path