Simple Perl script

#!/usr/bin/perl
use XML::WBXML;

# slurp mode
$/ = undef;

# read XML
$xml = <STDIN>;

# convert from XML to WBXML
my $wbxml = XML::WBXML::xml_to_wbxml($xml);

# convert to ASCII string of hex values
my $data = unpack "H*", $wbxml;

which is used something like

cat file.xml | perl wbxml.pl