Embedding a DTD within a DTD?

[AndrewSmall]

I am using the Open Travel Alliance (OTA) XML specification but need to send additional elements along with the standard validated XML. I have so far added an internal DTD subset along with the OTA standard external DTD subset. I would like to move the internal DTD subset into an external file of its own without modifying the standard OTA DTD. Can I create a new external DTD with my extensions that also embeds the OTA DTD? I have not seen any reference to this in any tutorials and think that this must be something that others would like to do. I.E. extend industry standard DTDs without modifying them.
How do I do this?

 

[AndrewSmall]

One of my coworkers found the answer. The DTD !ENTITY statement handles this.