add animation frames to existing spritesheet with animations?

If i have a spritesheet with some animations already made but then realize i actually need more animations which require new sprites, how i can i extend the canvas, add the sprites to the sheet and then re-slice it without breaking all the existing animations? a corollary to that, is it possible when slicing for it to leave the custom names i gave the sprites rather then resetting it every time? also, swapping out a spritesheet but using the same animations is impossible? i have potentially hundreds of characters all with the same animations, how do i deal with that?