To get the newer kopete, you could try porting it from the testing or sid sources to your stable system
. From the preceding link, I think there's one error, that being the step "$ apt-get build-dep foo" -- I think that this step needs to be run as root or sudo. I generally use the older method, described here
. Both work. These are methods to build a debian package for your stable system from the unstable sources.
Porting from unstable to stable is how I installed abby and cclive on my system. However, when I tried to do this with k9copy (I wanted the newer version), it required far too much work, and would have required me to pretty well alter everything, and thus destabilising my system. So, instead, I just installed the testing version, using a preferences file. My preferences file is as follows:
Pin: release a=lenny-backports
Pin: release a=stable
Pin: release a=testing
Pin: version 0.4.4-1
Pin: version 0.5.3-1
This way, I could maintain my system as stable, but install k9copy from testing (and maintain the abby and cclive programs as the ones I had ported). This worked. So, when I open any kde program I have, such as k3b, and press "Help - About KDE", it reads "K Desktop Environment. Release 3.5.10" whereas when I do likewise with k9copy, it reads, "K Desktop Environment. Version 4.3.2 (KDE 4.3.2)". Note, it's only k9copy that runs on the newer kde libraries, otherwise everything else is the debian stable version (that being, 3.5.10).
So, you can (in some circumstances) port a package from unstable to stable, or you can (with a preferences file) have some packages from testing and/or unstable installed while maintaining a stable version of debian.
Now my disclaimer for the second suggestion of mixed systems: I have an old system (a Pentium III), and I only use it for basic stuff. It may not be a good idea to mix stuff on a system where you do more intricate functions (IE, it's likely that having two kde libraries might play havoc with stuff like the KDE aRts module -- I use fluxbox, and just a few kde programs, so I don't really notice on my system -- IE, I don't ever have to run the two modules simultaneously -- I can keep them separate).