[XeTeX] Mapping inside font definition

[Mehdi Omidali]

Hi again,
I have a problem with an specific font that I couldn't resolve it by
fontmapping. I compiled the following lines with xelatex 0.999.6 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass{article}
\usepackage{arabxetex}
\newfontinstance\farsifont[Script=Arabic]{XB Zar}
\begin{document}
\begin{farsi}
1.1
\end{farsi}
\end{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
(please download the font from
http://irmug.org/portal/index.php?name=Downloads&req=getit&lid=30)
but I got another character as a thousand separator (instead of just a period .)
which is (I think) U+066C or U+E8C8. Now my question is this: how can I have a
period as the thousand separator in this font?
Thanks