Null byte injection using JSON

I’m trying to make a chatroom for my university, It takes username in JSON, and then stores it in an array, then takes it to DB for keeping logs, but the thing is, that array also has a "status" key, whose value is set to guest my default, but is set to ADMIN if I log in or any member from my team logs in. I know that the idea of storing "status" with username is bad but I just started working on the project. I want to confirm that is it possible to inject NULL byte using username field via JSON and add another key with same name "status" to gain admin privileges??