I haven't done this myself, ever, but I think
you should look into the following approach:
1) Leave the existing DTD as it is.
2) Create a new DTD document where you define your new elements.
3) Add an ENTITY reference to the original DTD file...
<!ENTITY old SYSTEM "http://www.mydomain.com/old.dtd">
...and
import it into the new DTD file:
&old;