How can I create an upload bundle containing sources and javadoc for a project?

You can do that by running the javadoc:jar and sources:jar goals before repository:bundle-create:

mvn sources:jar javadoc:jar repository:bundle-create

How can I create an upload bundle for a project that I don't build using maven?

You can manually add the artifact to your local repository, creating a POM and optionally adding source and javadoc jars. Then you can run mvn repository:bundle-pack that will prompt you for groupId, artifactId and version and will generate a bundle that you can use for an upload request.