Does anyone know if you can use the coupon in connection with a united trip to a location where nw and co don't have service?

I'm going to cambodia through Bangkok. I can ticket it on united using the bangkok fare which comes up to about $889 after all the miles trips. United then will let me add on thai air flights to cambodia (siem reap back from phnom penh) for another $150. But neither northwest or continental have partners in cambodia. Will that add on disqualify me for the discount?