if I could install debian-bsd such a way that I would add on my hard-dsk with other kernels a debian-bsd kernel to select that kernel from the grub menu, which I'm using for linux kernels, I'm using at present with 'sarge' debian... Or is it needed to install the whole thing on another hd?
You have to have dedicated (primary) partition for GNU/kFreeBSD. It is similar as for coexistence between linux and windows. All systems need private system partition, but data can be shared. Petr