This is the lazy and dangerous way to synchronize a directory with SVN including new files:
svn rm --keep-local dir
svn add dir
Although this can work in a pinch it can have serious consequences as well. For example, SVN will often lose track of a file's history.
The ideal way to syncronize a directory would be to be able to diff then use svn patch, however this deviates from formats in the diff command and the svn diff command will compare working directory differences rather than differences on the file system level.