seigex

Transit on separate itineraries is allowed, but there are caveats as you can't enter Japan. If are not on a Star Alliance flight to NRT, then it's very likely you won't be able to check bags through and you will have to be carryon only. Also low cost carriers like ZipAir will likely require you to go through security to landside to check in, which isn't allowed. So if you did something like ANA to NRT, then Avianca to the USA, you could check in through to the USA and check bags through and you're good to go.

Also keep an eye on connection times since you likely won't have protections on separate tickets as you would on a single ticket in case of a missed missed flight due to delays, so give yourself as much time as you need to be comfortable, especially if you're not familiar with the airport.