Hello Dennis,
Yes, you can add your own fileds along with user creation through API. Please have a look at this link for the code snippet.
P.S Information which you going to add in JSON Object (in key/value pair) is going to store in App42 No-SQL database. If you want to find these details along with get user method than please use this code snippet.
Let me know if you need any help from my side.
Regards,
Himanshu Sharma