First thing, you'll need administrative access to add an Attribute (IBA = Instance Based Attribute, means User Defined Attribute).
Second thing, you'll need to understand the difference between [Data Object] Type and an Instance. The type is essentially a blueprint that defines all instances. The instances are the actual Documents. I'm thinking this may be a key component of your frustration... in order to add an attibute to an instance you need to add it to the blueprint and as a result it will be added to all instances of the Type.
To find information on accomplishing this refer to the Help Center:
Specialized Administration > Tailoring Business Objects > Type and Atribute Management > Working with the Type and Attribute Management Utility and within that you'll probably want to have a look at Managing Global Attributes.
If you only want to add the attribute to certain types of Documents, consider creating a Soft Type (sub-Type) of WTDocument or Reference Document and adding the Attribute to the new Soft Type.. For this refer to Managing Types... again Admin access is required.
Your needs may be satisfied by a simpler approach though. Consider creating a Reference Document (instance) and naming it "Capability Study..." and relating it to the WTPart that is the subject of said Capability Study.
Reading your post, I was unclear on whether you were wanting to add the Attribute to a Document or Part.