Power Word: Shield

*it would be great to add spells yourself

Hey stenan2,
You can add Power Word: Shield by navigating to the following folder
...\World of Warcraft\_retail_\Interface\AddOns\BigDebuffs
and then open Retail.lua in some kind of text editor, search for the Priest section, and add the following line:
[17] = { type = BUFF_DEFENSIVE }, -- Power Word: Shield

Now that being said, I'd recommend against adding PW:S, as it is constantly being applied when playing with/against priests and is usually not worth trying to dispel outside of Rapture. Adding it to BigDebuffs might cause you to miss meaningful buffs/debuffs!