Adding extra fields in user table and accessing them

We want to add extra user fields in “user” table. And also want to access, update them. We don’t want to use user/custom API.
Please tell me how we can achieve this.

We found one issue in API doc while executing the Custom APis.
I added one custom field as “zipcode” . It got added for the current logged in user.
But when I tried to get the details in ID parameter neither the record is nor the userid works.
We have to put the filed name in ID field then only it retrieves the record. Find the screenshots attached.
So can you please clarify why there is mismatch in parametes.