If you meant to burn an ISO file as is, and if the ISO file dimensions is not too large to fit a CD, then it is possible. Simply you must create a data CD containing all ISO files.
If you meant to burn the content of ISO file on a CD and then adding the content of another ISO file to the CD, then it is not possible
The only way is to unpack all the contents of each ISO file on a separate folder first, and then creating a CD with all unpacked data.
You can use 7-zip or WinRAR to unpack all the contents of each ISO file.