Q. How can I export or import a Maven 2 format repository over HTTP into Nexus Repository Manager 2.x-3.x?
A. There is no repository export or import built into the product yet. We do have an issue tracking this feature.
However you should be able to use a third party tool to do this over HTTP using standard request paths.
Here is a third-party tool which does this:
Another option is the transfer_artifact command of the RiotGames nexus_cli.
Any problems with the above tools should be directed to the tool author.