The simplest way to export your multi-page whiteboard from Jamboard is to use the Image export feature for each page.
Follow the next instructions to export each Jamboard page and import into NotebookCast whiteboard.
1- Create a NotebookCast whiteboard and add the proper number of pages.
2- Go to your Jamboard and from the top three dots menu, choose the Save frame as image option, this will create an image for the current page, repeat for each page.