cryptkey and preferences.dat must match for amule to work, so if you modify one, the other should be modified also. I can only suggest you to use amule on a windows computer, generate several pairs of cryptkey.dat/preferences.dat files, and use one pair for every user. Although, is strange, since when I delete my .aMule folder, it seems to genereate a different userhash, even when using my same computer and even the same user.
Regards.