If you have a spreadsheet in one of the levels, you can save it separately from the rest of the session.
Once a spreadsheet is inserted, it will have menus right next to the level number. If you select the Table▸Save sheet as text menu, you will be prompted for a file name. You should choose a file name that ends in .tab. Once you save the spreadsheet, there will be a button to the right of the menus which you can use to save any changes you make. If you want to save the spreadsheet under a different name, you can use the Table▸Save as alternate filename menu entry. You can also use the Table▸Save as CSV and Table▸Save as mathml menu entries to save the spreadsheet in other formats.
You can use the Table menu to insert previously saved spreadsheets; the menu item Table▸Insert will bring up a directory browser you can use to select a file to enter.