The last versions of Ubuntu allow to download updates during the installation process.
Is it possible to configure a PPA to fetch updates instead of using the default repositories?
E. g. configure the Kubuntu backports PPA when installing Kubuntu 11.10 to get KDE4.8 during the installation process.
No, there is no there is no option like this in the Ubuntu installer (ubiquity) and I'm quite sure there is nothing like this in the alternate installer as well.
I believe your confusing things a bit. By default the highest version from all repo's is installed. You can set preferences to prefer one repository over another by using apt-pinning. Synaptic makes this very easy for you to do in a gui setting:
sudo apt-get install synaptic
On the menu after starting Settings->Preferences->Distrubution. Change to prefer versions from the kubuntu backports ppa.