If you’re already used to using PowerPoint, or already have skads of presentations already made there, but interested in making the transition to Google Slides, there is a lovely feature called “import slides” from which you can directly import specific slides from your pre-made presentations.
In addition, you can always upload your PowerPoints into Drive, and then right click on the file to tell it to “Open in slides”, at which point the whole presentation will be converted for you.
Amanda M. Orozco 