To acces a freebsd partition under netbsd.
Boot to freebsd and type disklabel /dev/.... (diskdevice).
FOr example in freebsd you also have a seperate partition for /home
Look with disklabel on which sector it starts and how large it is in sectors.
Remember that.
Be sure to unmount you netbsd /home (or any other you dont need for the
time being).
type "disklabel -e /dev/...." 
Uncomment the /home from netbsd there.
Add the /home with begin sector and size in sectors from freebsd.
As fs also use 4.2BSD of course.
After this mount /home and there is are, on the /home of freebsd.
Done it many times myself. It's only to bad that in netbsd
you can only mount 5 or 6 slices at one time i believe.
Bye,

Mipam.