I don't think it is possible to make it into a modification because there are very few hooks located around the AdminCP. A quick way would be to edit the options.php file if you are willing to.
Open options.php in a plain text editor then look for
PHP Code:
$optionsmenu = "\n\t<select name=\"" . iif($vbulletin->GPC['expand'], 'varname', 'dogroup') . "\" class=\"bginput\" tabindex=\"1\" " . iif(empty($vbulletin->GPC['dogroup']), 'ondblclick="this.form.submit();" size="20"', 'onchange="this.form.submit();"') . " style=\"width:350px\">\n" . construct_select_options($options, iif($vbulletin->GPC['dogroup'], $vbulletin->GPC['dogroup'], '[all]')) . "\t</select>\n\t";
It is around the line 1000 region so I recommend you use the "Find" feature. Add above that:
PHP Code:
asort($options);
Save it, upload it and you should be good to go.