le null se place automatiquement sur l'ancre de l'objet et non pas au centre de la composition

You cannot create a new null and have it automatically line up with another layer. But you can hold down the Shift Key and Parent any layer to another layer, and the Child layer will snap to the same position, scale, and orientation as the Parent layer. I use this trick all the time. I will often immediately remove the Parent and reverse it to make the Null the Parent so that it acts as a mover for other layers. 

Unless you use scripts that's not possible. New layers are always created smack in the middle of the comp or in case of text and shape layers where they are drawn. Just download one of those utility suites/ toolbars from AEScripts.com. Many of them have a "match position/ rotation/ scale" function.

Mylenium

Mylenium