Unfortunately this is the one remaining wizard that has not been converted to the new framework and so makes it a fairly involved process and requires changes to Alfresco source files. This will hopefully be adressed in the next release (2.2).
To make the changes you want you will need to add an extra field(s) on the JSP, in this case /jsp/wizard/new-user/user-properties.jsp. You will then ned to add the getter/setter methods to the backing bean, in this case that will be org.alfresco.web.bean.wizard.NewUserWizard. Finally, update the finish() method in NewUserWizard to add your properties.
NOTE: To edit the user details, you'll also need to update the populate() method.