DataNucleus supports persisting/retrieving objects to/from Excel documents (using the datanucleus-excel plugin) which makes use of the Apache POI project. Simply specify your "connectionURL" as follows
replacing "myfile.xls" with your filename, which can be absolute or relative. This connects to a file on your local machine. You then create your PMF/EMF as normal and use JDO/JPA as normal.
Things to bear in mind with Excel usage :-
Some references that may be of some use