Many times there is a need to break up HUGE files to make them more manageable and faster to download the pieces of files rather than one huge download.
I have used a couple of programs to combine and create ISO files of my own. The programs I use are PowerISO and MagicISO.
There are free versions to try, however the real benefits come from the FULL paid versions that allow large file importing and work with many iso formats in use today.
If you need a more detailed instruction of where to get and how to use these programs, let me know and I will make a Hub out of it.