Use this PAN Localization program to convert non-Unicode Khmer documents into Unicode. But you must install the whole package in order to get the stand-alone converter. This file will install, “five stand alone applications and one Microsoft Office plug in, extra fonts for Khmer conversion application, five stand alone java application and one plug in OpenOffice.org”
Download: PAN Khmer Font Encoding Converter for Windows
or
Download: PAN Khmer Font Encoding Converter for Linux