There was no expansionism. Maritime technology did not support trans-oceanic commerce or massive immigration. The colonists would have to be exiles or renegades who had a very high level of expertise in agriculture and craftsmanship, plus a supply of livestock and crops. Such a mission would be exceptionally unlikely, but not ASB impossible. Historically, technology propagates through a handful of exceptionally resourceful people. Perhaps a cadre of people who are perceived as threats to established order are ostracized and are able to propagate their skills in a new environment.
Perhaps we make a double POD. Chinese agriculture comes to the west coast. The Viking expeditions bring metalsmithing to the Great Lakes. Agriculture comes down the Missouri River and Metalwork down the Illinois, converging around Cahokia, uniting old world technology in the central US long before Columbus.
Maybe, but China continued to occasionally undergo widespread disorder from time to time, and those ultimately resulted in very gradual emigrations to Southeast Asia, which only totaled a few million over a millennia or so. In order for exiles to flee all the way to the Americas, China would have to undergo extreme disorder, specifically involving severe fragmentation at a level not seen since the Spring and Autumn and Warring States Periods (10+ entities competed over four centuries, and the country had remained divided from 771-221 BC) after the Song or so, given that maritime travel continued to be limited along rivers until then. This theoretical period of disorder becomes extremely unlikely after the Han (206 BC-220 AD), and virtually impossible after the Tang (618-907), due to gradual entrenchment of Chinese ideology (Tianxia, or "All Under Heaven").