I had the same concern as I'd rather have a ticket with
Genie+ included, especially to protect against G+ price changes.
I called today to see if it can be added on now and you cannot. Once a ticket is purchased without it, you have to wait to add it when you scan in the park daily.
But I figured saving $84 on a 3-day hopper for the
Disneyland Half is hopefully a bigger savings than a possible increase for Genie+. Genie+ would have to go up $28 more dollars per day for my savings to be completely canceled out.