If you're used to using GRUB, why not install it from Ports? IMHO, GRUB is a better choice than the FreeBSD bootloader, if you're going to be booting multiple operating systems. But if you insist, I think
boot0cfg is what you want, though I'm not sure you can get it to do what you want, because it's a pretty simple program. BTW, as far as I understand it, /boot/boot0 is the loader code itself, so you really don't want to edit this file.