You have two options:
- Use repositories import and point it to Archiva's storage.
- Configure Archiva as a remote repository for Artifactory, and run builds for a while. When you're confident all the artifacts were cached, take Archiva down and mark the remote repository offline.
The benefit of the former is a simple and one-time procedure. The benefit of the later is no downtime and import of only artifacts that are really in use.