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.
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.