Bug 37353 add variable button can't add variable only edit. [build path}
Project->properties->java build path->Library tab->add variable button will only allow you to extend or edit a variable not add a variable. The only way to add a classpath variable is to go Window->preferences->java->classpath variables->new button.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/NewVariableEntryDialog.java
String name= entries[i];
			IPath entryPath= JavaCore.getClasspathVariable(name);
			if (entryPath != null) {
				elements.add(new CPVariableElement(name, entryPath, false));
			}
		}
		
		fVariablesList.setElements(elements);
	}
	
	
	/* (non-Javadoc)
	 * @see Window#configureShell(Shell)
	 */
	protected void configureShell(Shell shell) {
		super.configureShell(shell);
		WorkbenchHelp.setHelp(shell, IJavaHelpContextIds.NEW_VARIABLE_ENTRY_DIALOG);
	}	
			
	/* (non-Javadoc)
	 * @see org.eclipse.jface.dialogs.Dialog#createDialogArea(org.eclipse.swt.widgets.Composite)
	 */
	protected Control createDialogArea(Composite parent) {
		initializeDialogUnits(parent);
		
		Composite composite= (Composite) super.createDialogArea(parent);
		GridLayout layout= (GridLayout) composite.getLayout();
		layout.numColumns= 2;
		
		fVariablesList.doFillIntoGrid(composite, 3);