I've never heard of nor seen any 3rd party repos for PCLinuxOS. I have however successfully installed Mandriva .rpms on a PCLinuxOS system. My advice would be to install kpackage
You will then be able to double click any Mandriva .rpm and perform a test install to make sure it will work. That way you don't end up hosing your system.
PCLinuxOS is a fork of Mandriva, I don't recommend you using .rpms from other distros such as SuSe or Redhat etc.
Other than going direct to the sites for the packages, I also use these: http://rpm.pbone.net/ http://plf.zarb.org/ http://rpmlinux.org/
I think Synaptic has an 'Alien
' mode for converting other packages.