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cdrecord -v -eject speed=4 dev=0,4,0 -multi cd-iso-image-file-1.iso 

The CD is left in a mode which will allow it to receive more data. To append data one must know where the last burn left off:

cdrecord dev=0,4,0 -msinfo 

This will give you the starting and ending track numbers. i.e.: 0,16774. Use this result in mkisofs.

mkisofs -J -r -V SecondBurn -o cd-iso-image-file-2.iso -C 0,16774 -M 0,4,0 directory-with-files-to-add 

The TOC from the first and second session will be combined.