Not really necessary. Just look at the /boot/grub/menu.lst that's on Fedora's partition and copy over Fedora's section into Sabayon's /boot/grub/menu.lst. You _could_ chainload, but it's not necessary. This is the easier way, imo.
Of course, you'll have to mount the partition in order to "see" Fedora's menu.lst. Let us know if you need help with that.
By the way, .... FEDORA ???