You have two options.
You can either install grub in your opensuse installation and install it to the root partition, NOT the MBR.
You can just make a /boot/grub folder with a menu.lst file that contains the info for your suse kernels.
when you update-grub in your mint install grub2 will then pick up the contents of your menu.lst file.
The first approach has the drawback of installing grub to a partition boot record using blocklists, which are said to be unreliable.
The second approach has the drawback of the need to manually update the /boot/grub/menu.lst file each time you upgrade the kernel.
I use the second approach on my Gentoo installation.
Originally Posted by gentoo /boot/grub/menu.lst
title Gentoo 2.6.38-rc6+
kernel /boot/kernel-2.6.38-rc6+ root=/dev/sdb8 ro
title Gentoo 2.6.38-rc5+
kernel /boot/kernel-2.6.38-rc5+ root=/dev/sdb8 ro
title Gentoo 2.6.37-gentoo
kernel /boot/kernel-2.6.37-gentoo root=/dev/sdb8 ro
title Gentoo 2.6.38-latest
kernel /boot/vmlinuz root=/dev/sdb8 ro
If you look at the last entry, you will see that I have an entry called latest. What I do here is use a symbolic link to point to the newest kernel. If you use an initrd you will also have to have a line for those
EDIT: I guess that you really have a third option. Which would be to cerate a custom entry in the
/etc/grub.d/40_custom file. If you would like to go that route let me know and I can help you with that.