Is it possible to export a master slide from one project to another?

Master slides are part of a theme. You can indeed copy/paste a master slide from one project to another project, but the object styles and (another component) and other design elements (like project background) will be changed IF that other project uses another theme. Just a warning. More info:

What's in a Theme - a Template? - Captivate blog

Open both projects to the Master Slide view and use Copy > Paste.  Ideally both projects should be of the same dimensions.