Here is a start, taken from
Pinochet's Shaman Guide:
#showtooltip
/use [@mouseover,harm,nodead][] Purge
As for using it with Shift, idk. You can put that on just about any key, besides Shift, Alt or Ctrl and it will work. The keybinding menu won't let you assign certain keys to the action bar because they are reserved as modifiers. I'm guessing what you are trying to work out is getting some sort of modifier in there and I can't help you with that because I really don't know much about macros. I just knew where the guide section was.
edit: There are macros that use modifers in them, and depending on which modifier is held down, it will cast that spell. For instance, you can have one macro that casts Wind Shear when you press the assigned key, and Purge when you press Shift and that key.
#showtooltip
/cast [target=mouseover,mod:shift] Purge; [target=mouseover] Wind Shear
I stole the syntax from some post on wowhead, but I put different spells in there, which you could also easily do. You still have to put the macro on your bar somewhere, and since it won't let you assign Shift to a button, you would have to put such a macro on another key. However, this sort of macro lets you consolidate the number of action bar buttons you would have to use.
I think I learned something today.