Doing this is a slight pain, to quote what Alan Storm said on StackOverflow Show archive.org snapshot :
So, if you really need to add a handle to a controller, you need to replace the call to $this->loadLayout()
with:
$update = $this->getLayout()->getUpdate();
$update->addHandle('default');
$this->addActionLayoutHandles();
$update->addHandle('your_handle');
$this->loadLayoutUpdates();
$this->generateLayoutXml();
$this->generateLayoutBlocks();
$this->_isLayoutLoaded = true;