The 4/3 offer has 7-day MYW base tickets unless you ask for 1-day passes.
You can add anything to those passes when you book your package or you can wait until later. So, if you want 10-day MYW hoppers, you can just ask for them when you call.
Ticket upgrades can also be done any time within 14 days of the first day of use. So, if you'd like to wait to see whether you want to pay for hopping, or waterpark fun or more days, then you can do it after you arrive at WDW.