configure GRUB to boot FreeBSD ?
 

Hello,

I have already Debian and Hurd. How do I add FreeBSD ? I imagine I have to manipulate /boot/grub/menu.lst ?

Thank you.

Yup. See this howto, step #3. The GNU Grub documentation may also prove helpful.

Hello,

is '/boot/loader' standard for FreeBSD ? I don't know which kernel or kernel loader to specify.

configure GRUB for FreeBSD
 

/boot/loader fits, you don't need to specify a kernel as you do for linux.