I did not find a correct way to perform the ActiveX code.
hdlDoc.CustomDocumentProperties.Add('ExtraProp',false,4,'NewValue');
Nevertheless, now I use a workaround. I wrote a word macro template (.dotm) with VBA code. It includes the Add-method with input variables. Then in Matlab, I use the ActiveX to load and run this macro.
%% Initiallize Word Connection
hdlWord = actxserver('Word.Application');
hdlWord.Visible = true;
%% Loading Macro Template
templateFile = [scriptPath '\MyMacros.dotm'];
hdlWord.Application.Templates.LoadBuildingBlocks;
hdlWord.Documents.Add(templateFile);
%% File and Location
docName = 'PropTest.docx';
fullLocation = [docPath '\' docName];
hdlDoc = hdlWord.Documents.Open(fullLocation);
hdlCDP = hdlDoc.CustomDocumentProperties;
%% Adding CDP
macroName = 'AddCustomProperty';
propName = 'ExtraProp';
propType = int32(4); % 2=Boolean.3=Date.5=FloatingPoint.1=Integer.4=String.
propValue = 'Hank!';
% Call the macro (now loaded from the template)
hdlWord.Run(macroName, propName, propValue);
Works fine. Downside: I always need this external macro file in the background.