How do I use the value of a webform_field as the default-value for a later webform_field?
The short answer is 'No' to using tokens to sync two element values, unless the two elements are on different pages. The simplest solution is a write a little JavaScript to sync the nickname with the username.

You can search for examples of sync'ing two input fields. 

The Webform module's Options admin widget uses a little JS to sync an option's value with the option's text. @see webform.element.options.admin.js