Just to keep interested forum readers informed:
From my understanding, to write a Quark XTension you need to become member of the QuarkAlliance, and that will cost at least 195 USD/Year.
On their website http://www.quark.com/en/partners/alliance/programs/xtensions.html I find
"As a QuarkAlliance basic-level developer you will receive the XTensions
Developers Kit (XDK) for all Quark software and time-limited versions of
software for your development efforts."