adding new element to divcomponent.mix

hi,
i'm trying to make a dtd based on docbook.
my own dtd contains the line :
<!ENTITY % DocBookDTD PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"docbookx.dtd">

%DocBookDTD;

i've found some documentation to add elements in *.class entities, but i'd
like now to create an element into the divcomponent.mix entity, but i can't
find a way to do it
does anyone have an idea?