Simple: How do I add data into custom field while creating new user with user_save
I had the a similar problem. This code helped me.
I put this code in myform_submit.

$fieldvalue = $form_state['values']['fieldname'];


//Loading the user object

$account = user_load($GLOBALS['user']-&gt;uid);


//Gettin the user data

$edit['data']['fieldname'] = $fieldvalue;


//saving the achieved data

user_save($account, $edit);


//Retrieving the user value

global $user;
$whatever = $user-&gt;data['fieldname'];