Launch Toast and select the "Image File" format from the "Copy" tab. Choose the created .toast image file by clicking the "Select" button or by double clicking the .toast file. Once the file is selected (a summary will display once the image is recognized) click the "Mount" button located next to the "Select" button. The disc image should now be mounted to the desktop. Select the "Data" tab, toast offers several data formats to choose from (ISO, MAC/PC, etc). Once the data option is chosen drag the mounted disc from the toast image into the project window. All of the content from the disc will added to the project window. This is where you would then add or delete files, in your case you want to add a file, so simply add the file to the project window. Toast will allow you to create a new disc image by selecting "Save as Disc Image" from the File "menu" or selecting the "Save as Disc Image" button located next to the Red recording button.
Instead of changing the extension on the image file, following the mentioned steps will create an ISO image file if "ISO" is chosen from the "Data" format.