You can always select DD EX (although, as Sanjay points out, PLIIx is probably the preferred tool for the job), but a Dolby EX encoded title is similar to a Pro Logic encoded stereo track back in the days of VHS - the surround channels of a Dolby EX track are mixed so as to facilitate the extraction of a surround back channel. My way of thinking about it is that an EX track is one that gives the EX processing instructions on how to generate the sixth channel instead of asking EX to figure out it on its own.