@ ajmarti: Some hair content are actually props which are smart-propped to 'fit' particular characters.  In these cases, you will need to parent the hair piece to the model's head.  Nowadays, hair will either be in its own category or as CR2 files under the Libray Figure section.

@mariajudyta:  What you are describing in your post is a dynamic dress which must be applied to your model in the Cloth Room.  The clothing has no rigging to properly conform it.  Read this tutorial to gain further insight about how to work with dynamic clothing:
http://www.renderosity.com/mod/tutorial/index.php?tutorial_id=2079&page=1

Practically, you would like to work with conforming content which is easier to apply and will move with your model when you change the pose.