What do I use to convert Northern EASE-Grid to Global EASE-Grid?
Assuming that the data that was produced conforms to the ML (25km) EASE-Grid definition, defined by the Ml.gpd and described in our 'All About EASE-Grid' documentation, then you should be able to utilize the IDL tools that are included in the EASE-Grid geolocation tools that we distribute. (available at: http://nsidc.org/data/ease/tools.html).
The IDL programs, "EASE_CONVERT" and "EASE_INVERSE" will translate to and from lat/lon and r/s coordinates for a given grid.
There is also an html-formatted help file that describes the IDL program interfaces in each geolocation tool's tarball called, "easeconv_help.html".