If you want to check out (part of) a repository into an existing repository with subversion then you need to use the svn:externals property. I can never remember how to do this, hence this page. What this means is that a piece of code which is under version control can be shared between multiple projects easily. This is relevant for, for example, work done with comedi and the common_comedi code we have created. Any changes to common_comedi can be propagated to all projects easily.
So, assuming you are in your checked out copy of the repository. You want to add the repository /home/ral/svnroot/common_comedi as a directory common_comedi within your checked out copy. This will place the common_comedi directory in the current directory.
svn propset svn:externals "common_comedi file:///home/ral/svnroot/common_comedi" .