Hi,
Well the add user frontend component was made to just add the user and then click the username in the 'created message'. Then you can fill out all fields.
However you can add as many extra fields as you wish in 3 simple steps.
Open the file: components/com_adduserfrontend/views/adduserfrontend/tmpl/default.php
1 - Go to line 284. (Add any type off input fields there in html)
For example:
// Paste this in between line 283 and 284 //
// change yourinput with your fieldname //
<tr>
<td>Fieldtitle: </td>
<td><input type="text" name="yourinput" value="" /></td>
</tr>
// Paste this in between line 283 and 284 //
2 - Go to line 31 (get the post data!)
For example:
// Paste this on line 30 //
// change yourinput with your fieldname //
$yourinput = trim($_POST['yourinput']);
// Paste this on line 30 //
3. Go to line 132 (insert the data in sql table)
For example:
// Paste this on line 132 //
// change yourinput with your fieldname //
// change cb_your_target_field to your cb field name //
`cb_your_target_field` = "' . $yourinput . '",
// Paste this on line 132 //
I hope this helps you

Kim