Edit /etc/grub.d/40_custom
#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
menuentry "PC-BSD 9.1 /dev/sdb3" {
insmod ntfs
set root=(hd1,3)
chainloader +1
}