This code adds the attribute to the project area, if it is not available

if (null == customString && fCreateAttributesIfMissing) {
		customString = workItemClient.createNewAttribute(fProjectArea,
				fCustomStringAttributeID, AttributeTypes.SMALL_STRING,
						IAttribute.FULL_TEXT_KIND_DEFAULT, monitor);
		customString.setDisplayName("Created String ("
				+ fCustomStringAttributeID + ")");
		customString = workItemClient.saveAttribute(customString,
				monitor);
}

if (!workItem.hasCustomAttribute(customString)) {
		// If not defined in the process template it has to be
		// manually created
		workItem.addCustomAttribute(customString);
}

Caveat of this approach was that the attribute was not available in the process XML as far as I remember. It was available for usage in the project, but did not show up in the configuration editor. I have not tried this for a very long time.

All the code needed to create new attributes and editor presentations is available in the Eclipse client. Out of curiosity I have looked at the project area editor to find out more. However, it is so abstracted out and really complex, that I gave up trying, because I did not have the business justification to spend days on it.