To just throw a bunch of iso files onto a disc:
growisofs -speed=1 -Z /dev/hda FIRST.ISO SECOND.ISO THIRD.ISO
To create a disc image of an iso:
growisofs -speed=1 -Z /dev/hda=IMAGE.ISO
Of course for the latter command you would need to combine the images into one image. You can mount them with something like: mount -o loop IMAGE.ISO /mnt/cdrom
and then copy everything to a directory and run mkisofs on that directory.
To store data on the "free space" you would need to have created a multisession DVD.