This is quite an old post with no response. It may be because you did not provide enough context or you may need to reword your question. I'll take a shot in case it helps you or others…
Custom metadata models can definitely be added. After defining your model you can configure the web client (either Alfresco Explorer or Alfresco Share or both) to show the custom properties.
You might be interested in this tutorial (2.x/3.x, 4.x) that describes how to do it.
