Thanks! Googling more about this, it seems the Russian portion of the ISS has 4 docking ports, but US portion has 2 docking + 2 berthing ports, and the Russian

Thanks! Googling more about this, it seems the Russian portion of the ISS has 4 docking ports, but US portion has 2 docking + 2 berthing ports, and the Russian ports are not compatible with the US ones. There is also apparently a US "pressurized mating adaptor" (PMA) that adapts a "common berthing mechanism" (CBM) port to a docking one, which sounds as if it might (if one were available, and could be installed) be able to adapt one of the US berthing ports for use by Dragon.