Often, users may want to create their own distinctive styles by creating a few
slide-masters. The user should be able to reuse his distinctive styles in the
new presentation he creates after that.

One way to do is to create a template. 

But sometimes the user may like someone else's presentation, and he may want to
use the same style.

In the Presentation Wizard, provide an additional option: Create a new
presentation with slide-masters from...

Let the user browse the presentation files, and select an existing presentation
file.

Impress should copy all slide-masters from the selected presentation in to the
new presentation.