1. Make sure you export your categories with file links in IMatch 3.
2. Make a copy of some of your image folders and index these copies with IMatch 5.
3. Import your category schema, using the CRC option to map the files.
The trick is to have the files in the IMatch 5 database before you run the category import. IMatch can assign files only when they are already in the database, not after the fact.