I won't say it is impossible, but KL upgrade availability to CO Onepass members is extremely limited. The best chance is to buy the KL ticket through CO, otherwise linking the KL ticket with your CO miles will be difiicult. Many agents don't know how to handle such a request.

Further, availability for LAX-AMS and AMS-ATH might differ from LAX-ATH. KL is famous for asking more miles for the first itinerary though.

There are some (limited) success stories which you can find searching this forum.

Good luck.