I believe I know where a converted narrow to long track Imp is being used currently. I will try to talk with the owner and find out what his conversion consisted of for the Imp.
A complete conversion to factory type specs would require axles, tracks, sprockets, cleats and a new axle housing as per mkntrakes post.
The quick way would be to just add new outer belts and cleats (asymmetrical tracks system) and use your existing axles and sprockets. I believe you would have to make or fabricate custom cleats for a track like this with an Imp. It may also become much harder to brake and steer with the asymmetrical tracks. Axle problems might also be a factor with this style track as more stress would be put on the axles when braking or turning.
Your best bet in my opinon would be to find an old junker wide track Imp and use the parts for your conversion or just find a cat suitable for your usage and sell your current narrow standard version. Finding repair parts for replacement is starting to be a problem on many of these older Thiokols.
Check out this great complete Imp conversion from a prior post for more info from nutsster on his conversion of a 1402 Thiokol Imp to a wide track:
http://www.forumsforums.com/3_9/showthread.php?t=3765