Whenever you add a New slide to a presentation, you'll be presented with pre-built layout options to choose from. Scroll down to find the option to Import a slide from one of your presentations. Just click on the presentation to see its slides and choose which one to import.
Below this, you'll also find the option to Import from presentations that have been shared with you or even from popular templates in Slidebean's template gallery.
The slides you select will be automatically added to the current presentation and will adapt the Theme you are working with.