No, it's not that common of a transition, but it can be done. The thing is, it's not guaranteed. One could apply to become a freighter FO after 12 months online (so about 15-16 months after Date of Joining) and be denied. Either way, if you come here as an SO be prepared for the possibility of spending some time in Hong Kong; right now upgrade to JFO on the Passenger fleet is around 3 years.
There are a variety of reasons that not many pilots go from SO to Freighter FO. Primarily, its financially driven. For the Europeans, Aussies, and Canadians (which make up the majority of the pilot workforce), there are HUGE tax advantages to living in Hong Kong-not that significant of a difference for US citizens. Plus there's the loss of the housing allowance by going back to the states. Also, for some it's the quality of life-SOs def have a pretty cushy schedule (as a friend of mine once jokingly said-he doesn't really ever want to upgrade to FO-it would get in the way of his social life!).
Whatever, to each their own. I will say though, that to truly enjoy the benefits of a career at Cathay, I would be in the mindset to spend more than just one or two years in Honkers, whether it be as an SO or as a Captain on the Pax fleet. HK is where you'll make the most money, and give one the opportunity to see a variety of places that you wont get flying out of the US.
Cheers