My 2 cents for what it's worth. Pellets are worst. Fresh hops are easier to maintain complete during a brew. Pellets tend to almost disintegrate, where fresh hops stay in whole during boil.
I would also suggest against it overall.
Also for what it's worth, the only way I thought of modifying a pack, would be to unseal the top (it's just glued together), add ingredients there, and close it back. This would ensure keeping the stuff in the pack, and filtered properly as long as it's not in powder. It's just an idea, I would certainly not recommend trying it.
I also wondered if I could use different ingredients that the packs don't offer. It's just not that simple. You really need a Z for this.
Enjoy your un-hacked packs! ;-)