If you have a PowerPoint file with multiple pages, you'll need to transfer them all, to ONE slide (page). As always, the best method is to directly type your information into the new presentation. But if you prefer to use content from another PowerPoint presentation, there are a couple of different ways to go about this. The first and quickest is the copy and paste method.

Create a new presentation in PowerPoint and set the page size to the desired dimensions. For help, see the section on poster size.
Open up your existing presentation and view it in slide sorter view.

Click on the first slide you want to put into your sign, and copy it to the clipboard (CTRL+C) as shown. When you click on the slide, you'll see it's border will be highlighted. In the example below, slide #1 is highlighted.

Switch to your new presentation, go to the menu bar and click on Edit>Paste Special as shown below.
Click on Microsoft PowerPoint Slide Object and click ok.

You'll notice in the paste special dialog box (top right), there are a number of different options. The Microsoft PowerPoint Slide Object will allow you to edit the contents of the slide after it's been pasted into the new presentation. If you were to choose Picture (Enhanced Metafile) or Picture (Windows Metafile) the slide will be pasted into the presentation as objects, and won't be editable. PNG, GIF and JPG will convert your slide object to a picture, those options are not recommended, the slide will not be editable, and will mostly likely print out slightly blurred or jagged.