I've worte a tool to migrate from Plesk 2 ISPCP. Cause iMSCP is a fork of ispcp, i think it could be helpfully to know, that i wrote this tool.
Its opensource, so have a look on it. If you do a migration, please tell me on sourceforgepage and here in the forum if its work, from which plesk version do you migrate to which iMSCP-Version and each other detail-Information which could be helpfully for other users.
Here is the link of the plesk2ispcp - Script: