DOCBOOK: ext file entities (with imported dtd)in sdbx

[Inge Juvkam <telmin@ol.telia.no>]

Hi all..
I hope i'll be forgiven for asking this question right away, 
-instead of lurking until it comes along.
My question is probably simple and i'll not beat about the bush...

How can i add entities/docs written according to another dtd?
(of my own design..) It is possible, or am i mistaken (again)?
I am writing a document in the _simple_ docbook for xml, it should 
be output to html and pdf or ps.
This should be ok to do with the stylesheets for the "real" docbookx?
If simpledb is a subset still after the alterations?

Does this all sound too trivial? Then please tell me..
Is there documentation i have failed to see on this subject
please don't be easy on me!
regards, inge