They're separate volumes, not partitions. I don't think you can do what you want with disk images. From the built-in Apple help (for Disk Utility):
Recording on a recordable CD more than once
Normally, you can burn items to a recordable CD, such as a CD-R or CD-RW, only one time. However, if you use Disk Utility to burn the disc, you can burn items to a disk in more than one session as long as space is available. This is also called multisession burning.
To burn a disc, you need an optical drive in your computer or connected directly to your computer. You can’t burn a disc using a remote optical drive.
To burn to a recordable CD so you can burn to it again:
In Disk Utility, create a disk image that contains the files you want to burn to the disc.
The files must be from a volume with a Mac OS Extended disk format. To check a volume’s disk format, select the disk in Disk Utility, and look at the information at the bottom of the Disk Utility window.
Select the disk image in the Disk Utility list, and then choose Images > Burn.
Select the “Leave disc appendable” checkbox. If you don’t see this option, click the triangle in the top-right corner.
Insert a blank recordable CD in the optical drive and click Burn.
To add more files to the disc later, follow the steps above. You can continue this process until all available space on the disc is used.