Edit /etc/grub.d custom with your favorite text editor and add this lines:
Code:
menuentry "FreeBSD" {
My root partition is /dev/sda2. If your root partition is different you must add for /dev/sdYX (hdY(a for 1, b for 2 etc..),gptX)
Code:
    set prefix=(hd0,gpt2)/usr/lib/grub2
    insmod ufs2
My FreeBSD's root partiton is /dev/sda4.
Code:
set root=(hd0,gpt4)
    chainloader /boot/loader.efi
}
NOTE: This tutorial for EFI systems.