The following will create a command which will create a new package to existing (and open) Java projects which have the same name as the Java project. For example if you have a project "de.vogella.test" in your workspace without any package this command will create the package "de.vogella.test" in this project.

Create a new plugin project "de.vogella.jdt.newelements".

Add a new command "de.vogella.jdt.newelements.AddPackage" and put it into the menu.

Create the following handler for this command.

			
package de.vogella.jdt.newelements.handler;

import org.eclipse.core.commands.AbstractHandler;
import org.eclipse.core.commands.ExecutionEvent;
import org.eclipse.core.commands.ExecutionException;
import org.eclipse.core.commands.IHandler;
import org.eclipse.core.resources.IFolder;
import org.eclipse.core.resources.IProject;
import org.eclipse.core.resources.IWorkspace;
import org.eclipse.core.resources.IWorkspaceRoot;
import org.eclipse.core.resources.ResourcesPlugin;
import org.eclipse.core.runtime.CoreException;
import org.eclipse.jdt.core.IJavaProject;
import org.eclipse.jdt.core.IPackageFragment;
import org.eclipse.jdt.core.IPackageFragmentRoot;
import org.eclipse.jdt.core.JavaCore;

public class AddPackage extends AbstractHandler implements IHandler {

	@Override
	public Object execute(ExecutionEvent event) throws ExecutionException {
		IWorkspace workspace = ResourcesPlugin.getWorkspace();
		IWorkspaceRoot root = workspace.getRoot();
		// Get all projects in the workspace
		IProject[] projects = root.getProjects();
		// Loop over all projects
		for (IProject project : projects) {
			try {
				// Only work on open projects with the Java nature
				if (project.isOpen()
						&& project
								.isNatureEnabled("org.eclipse.jdt.core.javanature")) {
					IJavaProject javaProject = JavaCore.create(project);
					IFolder folder = project.getFolder("src");
					// folder.create(true, true, null);
					IPackageFragmentRoot srcFolder = javaProject
							.getPackageFragmentRoot(folder);
					IPackageFragment fragment = srcFolder
							.createPackageFragment(project.getName(), true,
									null);
					System.out.println("Juhu");
				}
			} catch (CoreException e) {
				e.printStackTrace();
			}
		}
		return null;
	}
}