Adding slides to an existing presentation can be done easily with the help of the Outlining Toolbar.

 

From left to right :
Promote : This tool places a bullet text and its components into a new slide.
Demote : This tool removes a slide and places the contents as bullet text.
Move Up : Moves a section of text up one level.
Move Down : Moves a section of text down one level.
Collapse : Hides a slide's text components, showing only its heading.
Expand : Shows both the slide's heading and its text components.
Collapse All : Shows only the slide headings for the whole presentation.
Expand All : Shows both the slide headings and their text components (for the whole presentation).
Summary Slide : Creates a summary slide for the presentation.
Formatting : Shows/hides the formatting of text.