BSA — how to extend BSA DTF?

The question is how to extend BSA metadata to be able to include
additional vendor-specific tags.
Originally, I have suggested tag <extension> of type ANY to be used for
these specific tags. However, in that time my understanding of ANY was
wrong. I thought that an element of type ANY can have any data including
new tags. Which was a wrong assumption. One can use only tags (any of
them) defined in the DTD.
So now my understanding is that if I need a vendor-specific tags, I
need to extend DTD. The XML books say that an internal DTD has priority
over an external DTD and that document authors can override things using
their own internal DTD. Is this the way how to extend BSA metadata?
I have tried. Here is an example of an extended XML file with an
internal DTD defined new tags:

<?xml version = "1.0"?>
<!DOCTYPE DsLSRAnalysis SYSTEM "http://localhost/openBSA/docs/DsLSRAnalysis.dtd" [
<!ELEMENT extension (part1?,part2?,part3*)>
<!ELEMENT part1 ANY>
<!ELEMENT part2 ANY>
<!ELEMENT part3 ANY>
]>

<DsLSRAnalysis>
<analysis type = "search.list">
<extension>
<part1> this is part 1 </part1>
</extension>
</analysis>
</DsLSRAnalysis>

Unfortunately, this does not pass validation tests. I am getting an
error saying "Error at (file http://....DsLSRAnalysis.dtd, line 13, char
25): Duplicate element name, "extension".

Any ideas what I am doing wrong, or what is the correct way how to
extend BSA metadata? Is the element <extension> in BSA DTD useable at all?
If yes, how can be used, if not, should we re-consider it in the FTF?

Accepted. A valid DTD was defined.