Migration utility will be available when OpenKM 3.0 reach the final release. Actually is in RC1 (Release Candidate 1) stage. The migration procedure is based on repository structure changes and I want to be sure that repository structure is consolidated. The final release will be available in 4-5 weeks... I hope :) It depends on users feedback needed to fix bugs.
Anyway if you want to test this release with your stored documents, you can export repository of OpenKM 2.0 documents to filesystem and import them in OpenKM 3.0-RC1.
OpenKM Team ( Be open, my friend! )