Why don't you want to install GRUB on the MBR? If you're installing it beside another operating system, GRUB will pick it up and still give you the option to boot into that other operating system. Alternatively, you can choose to not install GRUB at all, and let the other bootloader handle things, though this may take some configuration in the other OS. The easiest and most common thing to do in a dual-boot situation is to just let the new bootloader nuke the old one and replace it.
That said, what you want to do probably won't work, anyway. If you could do what you wanted, then you'd end up with two bootloaders trying to fight for control. That's part of why there's a Master Boot Record (MBR) to begin with.
If you want to install Ubuntu, without wiping out your other OS, that's entirely doable in a number of different ways:
- Repartition the drive the other OS is on.
- Install on a different internal drive.
- Install to a USB flash drive and use the "Boot to USB" option in your boot options at POST time to boot to it the same way you'd boot to a CD.