I don't use Digital Editions or Kobo, so i don't really know. However ACSM Files – What they are and how to work with them « eBook Reader Software
might be of interest. If the books are DRM protected you probably can't usefully back them up, but I assume you can re-download them from wherever you originally bought them.
I think Aldiko can read all the formats (except MOBI) that Kobo can, so if you back up the books from our Kobo you should be able to import them to Aldiko.