The US PHEV does have a 12 volt battery in the right rear of the cargo area. I've seen a picture of the cover, just like the original 2016/17 European models. I've not yet discovered a specification yet that state it is lead acid (likely, but why) versus lithium. It is likely a low capacity battery either way. Either way, it shouldn't prevent you from modifying it as a portable power source. Obviously, the Prius folks are the experts on that, particularly those that convert regular hybrids to plug ins. There are shops in California that specialize in doing that so should be able to deal with a Niro.
2018 Kia LX HEV Metal Stream with Advanced Tech