My drummer wants to see the melody as well as his part on his chart. How 
can I do this?
Should I create another staff on his part and copy and paste the melody 
on that new staff? Would Sibelius allow me to create another staff on a 
part? Basically, I want to change the drum part, but I don't want those 
changes appear on my score.

Here is how I would do all this. In your main score moove to the drum part
by pressing "control+alt+tab" until you hear drum or whatever name your part
has. Now turn on the part window with "control+alt+r" or pressing menu
"menu/window/parts". With "control+windows+tab" moove into the parts window.
Tab to parts in window and press enter. In this dialog check the part you
want to add to your drum part and press enter. Still in the parts window tab
to extract parts for saving your part to a new file. In this new file you
can then make all your changes without compromizing your main score.