Assuming that your decorator class myLib_Form_Decorator_Lalala
is stored in file library/myLib/Form/Decorator/Lalala.php
, then we can do it as follows:
At the form level:
$form->addElementPrefixPath(APPLICATION_PATH . '/../library/myLib/Form/Decorator', 'myLib_Form_Decorator_', Zend_Form::DECORATOR);
At the element level:
$elt->addPrefixPath(APPLICATION_PATH . '/../library/myLib/Form/Decorator', 'myLib_Form_Decorator_', Zend_Form::DECORATOR);