Is it possible to have a button in a slide that can launch another file?

It is very easy to do this with PowerPoint and iSpring. On the PowerPoint toolbar, choose the "Insert" tab, then "Shapes" (or any other object you like), draw a new shape on the slide, and then specify the hyperlink or any other external file for this shape.

To add a hyperlink, right click on the shape you chose, select "Hyperlink" and specify the relative path to the file you want. Relative path means that the path only points to a file or a file path, like "my_video.swf". It does not include the full address of the file. 

Now click "Publish" and convert your presentation to HTML5 with iSpring.

When viewers click the button you created, the specified file will be launched in a new window.

Please note that both the generated presentation file and the external file you linked should be located in the same folder.