there is only the help page. But to make things easier, you can start directly by looking at the .layout file.
Open the JabRef.jar file with a archive manager. Then extract/copy the contents of the folder:
resource\layout\openoffice\ to a place on your filesystem.
Then open the openoffice .layout files with a text editor. You have to modify the .begin.layout file to add the Column name, e.g. DOI at the right position where you want to display it.
Then open the other layout file. At the right position insert the following snippet:
Save your changes. Then go to Options -> Manage Custom Exports -> Add new, enter a namem e.g. CSV with DOI
add .csv as extension. and save.
It should then be avaiable in The Export Menu Dropwdown box.
You an also apply a formatter-