It's possible, but I never tried it. You can use the same script as the CD does to install the Pluto packages on top of Debian and it will be almost the same as installing with the CD.
First, add this line to /etc/apt/sources.list:
deb http://deb.plutohome.com/debian/ 20dev main
apt-get install pluto-install-scripts
This will install some scripts in /usr/pluto/install.
Edit /usr/pluto/install/Initial_Config_Real.sh and comment out the "read" and "exit 1" on lines 108 and 109 since two packages that were considered critical in the past but are disabled now (didn't remove them though) aren't present on your system and will cause the script to fail if you skip this step.
Run /usr/pluto/install/Initial_Config.sh to begin the installation.
From here, everything will go as with regular Pluto.
This is needed only if you have a i686 non-compliant machine (i.e. 586 or older, like AMD K6-2, VIA C3 and Samuel 2 [but not Nehemiah, which is i686])
-- i386/i586 specific --
It will also install kernel 2.6 for 686 machines. Proceed like this, but when you get to "installation finished, press enter to reboot", don't reboot yet. Go to another console and type this:
apt-get install kernel-image-2.6.10-1-386
This will install the 386 kernel. You won't be able to use the ivtv or cx88 drivers with it since they're built only for the 686 kernel but everything else should work ok.
-- end of i386/i586 specific --
I don't know what to expect regarding the interaction between Pluto and the Sarge packages yet, since you installed a clean Debian there, but there should be no problem at all. We're upgrading our packages to Sarge after the 188.8.131.52 release, so any pre-upgrade feedback is appreciated. Thanks.
Initial data shows that it should work as expected with Sarge packages, the only problem for now is automatic upgrade from our snapshot to Sarge, which affects our current systems, not Pluto-over-Sarge installs.