No easy way to install new Kubuntu into already existing grub

[Ped]

I have at home already Kubuntu 6.10 and 7.04 installed, each of them on their own ext3 partition with shared ext3 "/home" partition.
I have the graphical grub at /boot/grub at Kubuntu 6.10 partition.

I tried to install Kubuntu 8.10 there, but from previous experience I think it will either overwrite the bootloader completely, i.e. making the grub from /boot of Kubuntu 8.10 partition (another one) active, or when I check off the setting in "advanced" options of installer, it will not touch the active grub at 6.10 partition at all, leaving me with installed system without boot setup.

As I'm not grub guru, by editing the menu.lst by hand and adding the partition/files of 8.10 I got into situation when booting up 8.10 brings some early error message about vmlinuz file missing? (I didn't had time to write down the error message, and it's not related to this wish anyway).

So I would appreciate a simple way in installer to (while installing new kubuntu at new partition) add the new Kubuntu into already existing grub at different partition (which should be left intact in any other way, just the /boot + /boot/grub files updated).