First of all make sure you have the latest ImgBurn version.
Copy all the files from the disc to a folder, add the files you want to be on the disc, then use ImgBurn's Build mode -> Advanced tab -> Bootable Disc tab to extract the boot image from the source disc, and when ImgBurn asks if you want to use that information answer Yes. Add to the Source list the folder containing the files you've copied from the original and click the big Write button