Once my content is imported, can and add content to an existing slide in the future and if so How?

Yes, you can. When you play your lessons, at any time you are able to add a blank whiteboard, annotate and add images. You can then save your content too. If you then want to add more slides, you click on the “open folder” icon which takes you to your workbooks. Click on the preview of your lesson and it will open in the editor.