[1.11]
Paste this into a command block set to:
Repeat | Unconditional | Always Active

then instead of PlayerNameHere put your IGN and it should work
(It also works for stuff other than snowballs)

/replaceitem entity PlayerNameHere slot.hotbar.8 minecraft:snowball 1 0

now you can have snowball fights without running out of snowballs!

To disable the command but still have the block, just set to Impulse.


Have fun!