I have observed your requirements and like to share that Aspose.Slides allows you to add the macros inside the presentation. I request you to please visit this documentation link to serve the purpose on your end. As far as your requirement of converting PPTX too PPTX is concerned, I like to add here that you can load the PPTX and save that as PPTX. The following sample code will serve the purpose to you in this regard.
Presentation pres=new Presentation(“test.pptm”);
I hope the shared information will be helpful. Please share, if I may help you further in this regard.