You can hack the core as follows:
in qa-include/pages/message.php
change this at line 52..
if ($handle === $fromhandle) {
// prevent users sending messages to themselves
$qa_content['error'] = qa_lang_html('users/no_permission');
return $qa_content;
}
to this...
// prevent users sending messages to themselves or user is not equal to admin
if ($handle === $fromhandle || $fromhandle != 'admin user name goes here') {
$qa_content['error'] = qa_lang_html('users/no_permission');
return $qa_content;
}