Use the Nexus artifact upload page (note this only works for multiple artifacts with the same groupId and artifactId).
Set up a script, with multiple invocations of the maven-deploy-plugin's deploy-file goal, one for each artifact.
If you have access to the file system, you can copy the files directly into [sonatype-work]/storage/[repository-name]. If you do this, set up scheduled tasks to rebuild the metadata and reindex the repository.