You can import your slides into Glean. You can do this at any time; before you start, during or after recording. Glean supports Power Point and PDF slides.
You will have to be on-line to import Power Point files (you can go offline once the file has been imported). PDF files can be added whether you are on-line or off-line.
If you have a different format from a different application, then you can usually use that application to Save As PDF or Export as PDF which will allow you to import into Glean.
Please see the articles below for more detail: