If you need to include a full Google Slides Presentation into Demoflow without importing it in, you can always publish the Slides Presentation and include the published link in Demoflow. Here's how:
Publish the Google Slides Presentation
Go the presentation you want to include
Go to File
Publish to Web
3. Make sure that you restrict access to only your organization (if applicable)
4. Hit Publish
5. Copy the URL
Add to Url to Demoflow
Create a new URL with that link, and viola! You've got a url with the full presentation.
