The created pom.xml will also have dependencies on any plugins they require.
Add the Eclipse RCP artifacts to your internal repository
This will add the Eclipse RCP artifacts which you have downloaded and install them into your internal repository external_free so they can be shared. These files need to be deployed to a maven repository so that they can be referenced in your pom as dependencies. e.g. #Create an Eclipse RCP Target will only be able to automate the creation if they are available in a maven repository.
This will also download all transitive dependent plugins.
Build your Eclipse Plugin
As far as I know the maven-eclipse-plugin can't help you to build your Eclipse plugin.
There is some work going on with OSGi bundles being built, some discussions with the Eclipse team to expose and unify classpath and PDE features to make it easier to externally tool plugin development.
However you can use the pde-maven-plugin@codehaus to build your plugins with Maven. pde-maven-plugin invokes the Ant pde build files to actually do the work. You can even create an RCP application this way.