So not true! Well only part, I've only known map sets to be in img format named gmapsupp like previously stated. I currently run 2 map sets on my Garmin Mobile XT, North America 2010 and Mexico 2010, and I accomplished this by naming the second map set gmapsup2. As for the voice you need to rename to one of the excisting ones in the voice folder. After doing so place in the voice folder overwriting excisting one, voice folder is located in your Garmin folder.
P.S. Done the same on my wife's nuvi so it works.
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