pptx to potx conversion is a matter of making a new template for existing presentation in PowerPoint and you can easily do that with the default save as.. function in the software. Some 3rd party PowerPoint alternatives might be of help as well, just keep in mind that not every presentation software might support latest version of PowerPoint formats.
Open PPTX file Open POTX file
Free presentation program from OpenOffice.org
Powerful multimedia presentation software tool from LibreOffice suite
A presentation program from the Microsoft Office suite
Mac version of popular presentation software from Microsoft