You can quote DocBook code in Org files and copy it verbatim into exported DocBook file with the following constructs:
#+DOCBOOK: Literal DocBook code for export
#+BEGIN_DOCBOOK All lines between these markers are exported by DocBook exporter literally. #+END_DOCBOOK
For example, you can use the following lines to include a DocBook warning admonition. As to what this warning says, you should pay attention to the document context when quoting DocBook code in Org files. You may make exported DocBook XML files invalid by not quoting DocBook code correctly.
#+BEGIN_DOCBOOK <warning> <para>You should know what you are doing when quoting DocBook XML code in your Org file. Invalid DocBook XML may be generated by DocBook exporter if you are not careful!</para> </warning> #+END_DOCBOOK