JabRef's export and import capabilities can be extended quite
  easily. Please unzip the JabRef-extensions file, preferably to the
  same folder where you installed JabRef (where the jabref.jar file can be found).
  
To create your own export format, you can define custom layouts,
  simple templates where at certain places the contents of a JabRef database are written
  into. For an example see the files in the layout directory.
  For more details about creating your own export format, please read 
  API/CustomExports.html.
The extensions contain also a few simple examples on how to create new field formatters -
  small commands that determine how database entries are written to your
  custom layout, please check net/sf/export/layout/format.  
  
You can also quite easily  create an importer that allows you
  to read references from sources important to you. This requires only
  modest programming skills. Please check the 
  example, compile it
  using the compileAll.bat script, register it
  as a custom importer with your JabRef-installation and
  import the SimpleCsvImporter.csv
  file to see how it is done. More details can be found
  in API/CustomImports.html.
All importers that are part of the standard JabRef-distribution are also
  included in this extensions-package under net/sf/jabref/imports. 
  In addition to serving as further examples, you can also fix them and register them as custom importers
  if you encounter any problems.
  
The API-documentation for extensions that require some programming
  is available under API/index.html.