Can report success so far.
Procedure:

  1. Installed Debian 6.0.1 w/LXDE-openbox: A
  2. HD-Installed Knoppix 6.4.4 32-bits kernel CD version: B
  3. HD-Installed Knoppix 6.4.4 64-bits kernel remastered DVD version: C
  4. Compared packages, installed 64-bits versions of all available packages installed in B on A
  5. Set up a 18GB partition with Reiserfs: D
  6. Transferred B-system to D by rsync
  7. Updated D-system from A by rsync
  8. Updated /boot on D from C: 64-bits kernel as vmlinuz
  9. Updated D-system /etc, /lib/modules, /usr/src from C by rsync
  10. Updated D-system /etc/alternatives from A by rsync. (To get the 64-bit library links right.) Added grub entry for D in common /boot, using basically copy of C-entry for D

So, essentially, this was carried out by a mix n' match of three OS installations, resulting in a fourth. If 64-bits support had not been broken in CD version of Knoppix 6.4.4, it might have been enough with two, but it was quite nice to be able to check with the full Knoppix version on HD. And starting with the CD version gave me fewer packages to install (or miss) in 64-bits versions.

There are a few minor editing details left out in the above list.

Here are the grub entries:
Code:
# entry created by 0wn

title KNOPPIX 64-kernel HD install DVD
root (hd0,7)
kernel /boot/vmlinuz root=/dev/sda8 rootwait lang=us apm=power-off nomce libata.force=noncq tz=localtime loglevel=1 ramdisk_size=100000 lang=en keyboard=no  nosound vt.default_utf8=0 apm=power-off initrd=minirt.gz nomce libata.force=noncq loglevel=1 tz=localtime rw

# ---

title KNOPPIX 64 FULL Devel vs 2
root (hd0,5)
kernel /boot/vmlinuz root=/dev/sda6 rootwait lang=us apm=power-off nomce libata.force=noncq tz=localtime loglevel=1 ramdisk_size=100000 lang=en keyboard=no  nosound vt.default_utf8=0 apm=power-off initrd=minirt.gz nomce libata.force=noncq loglevel=1 tz=localtime rw
As for the Knoppix vs Debian questions: No, Knoppix is not Debian. Knoppix is far, far better. Basic Debian is still stuck in the "being everything to everybody" sure-to-fail paradigm. That is one important reason why Ubuntu has strayed unneccesarily far from its parent.

Installed system size is now 5.8 GB, that is with full 64-bits R and VMware workstation and MySQL installed. Plus lots of unneccessary Debian 6.0.1 stuff - but it does no harm, I think. Typically, the 64-bits versions are somewhat bigger, I think the compressed kernels: 3.2 vs 2.9 MB, may be representative.