I am trying to customize a metal door frame, for masonry use. I need the frame to be 2" and I need the header to be 4". I noticed that this is not possible with editing and existing door. Hpw do I do this?
You can do this with a DWA (door window assembly). You can even define the profiles of each frame should you wish to.
As was correctly mentioned, the DWA is the way that AutoCAD Architecture wants you to go. To help you out, Autodesk has provided some doors like you describe with 2" frames and 4" heads. You can find them in the Content Browser under "Design Tool Catalog" -> "Doors and Windows" -> "Door and Window Assemblies". A couple of your options in there are "Hinged Double 4 Inch Head" and "Hinged Single 4 Inch Head".
(The Content Browser is the the master library of styles provided with AutoCAD Architecture. It is accessible from the Home tab and the Build panel, and the Tools dropdown. You can just drag styles directly from the Content Browser into your drawing.)
Those OOTB styles should get you started with what you desire.
I hope that helps you.