How to install a single package from Debian sid?

What you REALLY want is to learn about apt-pinning. http://jaqque.sbih.org/kplug/apt-pinning.html

This package is available in Debian Backports, which will make your life a lot easier. These are generally packages pulled from testing or unstable (sid), and "backported" to the current stable release.

Add this line to your sources.list

deb http://backports.debian.org/debian-backports squeeze-backports main

then run:

apt-get update && apt-get -t squeeze-backports install ibus-sunpinyin