Did you know you can add an existing PowerPoint presentation to yours and keep its format? This process may not be intuitive for many users.
With your main PowerPoint presentation open, go to the Home tab.
Click on the New Slide drop-down. Navigate all the way to the bottom of this menu and select Reuse Slides.
The Reuse Slide menu will appear in the right of your existing slide.
Click on the Browse button to navigate to the PowerPoint presentation you want to add to the current deck.
Once you navigate to the file, it will open in this menu.
There is a checkbox all the way at the bottom that says Keep Source Formatting. Check this in order to maintain the design of this presentation.
You can then double-click on each slide to add it to your current presentation.
Remember you can always drag slides to different areas of your presentation in the left hand panel if you make a mistake in the order.