6
           
            
                
     
    
    I think you have to replace:
 case "textbox": 
                //proceed to next cases 
            case "textarea": 
            case "dhtml": 
                $form->addElement(new XoopsFormText(_PROFILE_AM_MAXLENGTH, 'field_maxlength', 35, 35, $field->getVar('field_maxlength', 'e'))); 
                $form->addElement(new XoopsFormTextArea(_PROFILE_AM_DEFAULT, 'field_default', $field->getVar('field_default', 'e'))); 
                break;  
in /modules/profile/include/forms.php by
 case "textbox": 
                //proceed to next cases 
            case "textarea": 
                $form->addElement(new XoopsFormText(_PROFILE_AM_MAXLENGTH, 'field_maxlength', 35, 35, $field->getVar('field_maxlength', 'e'))); 
                $form->addElement(new XoopsFormTextArea(_PROFILE_AM_DEFAULT, 'field_default', $field->getVar('field_default', 'e'))); 
                break; 
            case "dhtml": 
                    // xoopseditor 
                break;  
And then add a wysiwyg xoopseditor as layed out in /class/xoopseditor/sampleform.inc.php .