Yes, you should definitely take a look at Marvin.
Kobos can read the same EPUBs that are already on your Nook, no conversion necessary. However, if the books have DRM, you will need to remove that first.
You may wish to take a look at
calibre for managing your books (including metadata management synced to the Kobo), and the
Kobo KEPUB driver for adding on-the-fly Kobo-specific enhancements to EPUBs.
For more information regarding removing DRM, see
http://apprenticealf.wordpress.com.