In the documentation of kdar 2.0.7 the configuration options -> directory filters there's written this:
Note, you might want to "Include a directory". In that case, only those directories that are included will be a part of the new archive. You can then exclude directories below those included directories.
Directories can be filtered using wildcards (known as "globbing"), i.e. "*", "?", "[", and "]". For example, to exclude all CVS directories from your archive enter "*/CVS/*" for the directory filter.
Along the same lines, you can include or exclude a specific filename by specifying its full path in the new directory dialog. If necessary, browse your filesystem for specific files by clicking the directory icon to the right of the input box.
The problem is that there are only directories but no files displayed in the file browser which is opened after clicking the directory button. So it's not possible to select single files.
This should be possible as far as dar supports this because sometimes it can be useful not to backup a complete directory but only a single file. Or it can happen that e.g. in the home directory not every subdirectory shall be backuped but the files which are stored directly in the home directory itself.