Are you talking about gathering your own user info? Do you want this data to go in the JSON file?
Well it looks like you could just use update profile snippet, which will let your users add or change info.
However I see two potential problems. First is, you *may* (not sure) have to use register snippet to make a new field for user data (maybe update profile can do it also), and making new fields on the fly will require some sort of custom coding, I think, but it might not be too hard.
I get some extended fields for users on register, and here is the sample code, this will take the data and put it into the JSON file:
<!--Date of birth-- If you want it in the User General Information field than "date_of_birth" must be replaced with "dob"-->
<li><label for="date_of_birth">Date of Birth:</label>
<span class="error">[[!+reg.error.date_of_birth]]</span>
</label>
<input type="text" name="date_of_birth" id="dob" value="[[!+date_of_birth:date=`%Y-%m-%d`]]" /></li>
To be clear, by giving this the name="date_of_birth" it becomes an extended field, if it was name="dob" it would go into the general information tab.
HTH
☆ A M B ☆