Actually, I've found a method to add view paths to individual form field widgets (formwidget) on the fly:
In this example, we add the partial path "plugins/vendor/plugin/controllers/myclass/myformfield" to the widget, which is responsible for the field "myformfield".
class MyClass extends Controller
{
public $implement = [
'Backend.Behaviors.FormController',
];
public $formConfig = 'config_form.yaml';
public function create()
{
$form = $this->asExtension('FormController')->create();
// add a new partial view path for a single form field
$companyFieldWidget = $this->asExtension('FormController')->formGetWidget()->getFormWidget('myformfield');
$companyFieldWidget->addViewPath('$/vendor/plugin/controllers/myclass/myformfield');
//
return $form;
}