TL;DR No, you can't. Here is me setting Smart Cast Spell 1,2,3, and 4 as Q,W,E, and R, respectively.
Now when I try to set Cast Spell 1,2,3,and 4 as [Shift]Q, [Shift]W, [Shift]E, and [Shift]R, this is the message I get. > Cast Spell ( ) cannot be bound as specified.
Key Bindings seems to only accept numbers and letters, in addition to some symbols.