Hi Imran !
Thank you very much for your help.
Meanwhile I did some more digging into this forum and I found some answers. From what I found, it looks like it is possible to add layers to a page, which is good for me.
I found this thread here:
With the following example:
Page page = dia.getPages().getPage(“PageTemplate”);
Layer layObj = new Layer();
long shapeId = page.addShape(3.0, 3.0, 0.36, 0.36, “Shape1”);
Shape shape = page.getShapes().getShape(shapeId);
I checked it and it seem to work fine.