While Sonatype Nexus Repository OSS does not have any UI-level support for moving an artifact between two repositories, you can very easily do this by just manipulating the underlying filesystem and reindexing the two hosted repositories. There are a few reasons you might want to use this functionality, maybe you want to support a promotion strategy between two hosted repositories? If this is something you find yourself doing, my first suggestion is that you consider the Staging functionality available in Sonatype Nexus Repository. If Staging isn't an exact match for your requirements, this post will detail the process you should follow if you need to move an artifact between two repositories.
The process for moving an artifact between two hosted repositories:
While the method documented in this blog post will work, if you find yourself frequently needing to move artifacts between repositories, you will want to look into Sonatype Nexus Repository, which offers a Staging Suite which provides this functionality with an easier and more intuitive interface. While Staging doesn't specifically move artifacts between hosted repositories, it provides for the ability to expose "staged artifacts" in a repository group and to either discard or "promote" an artifact to a hosted repository depending on testing and validation.