Greetings Michael!
How exciting to know you'll be arriving at the
Disneyland Resort soon to celebrate
Mickey's Halloween Party!
Guests of Mickey's Halloween Party don't need
MaxPass during the Spooktacular festivity, especially if you're planning on spending more time at Disneyland than at Disney California Adventure.
I say this because the FASTPASS service will not be available at the Disneyland Park during the Party. Also, your Mickey's Halloween Party ticket comes with free PhotoPass Downloads of all your Halloween pictures!
If you'd like to add MaxPass to use at Disney California Adventure (you must try Guardians of the Galaxy: Monsters After Dark!), you can certainly do so, however, you'd have to add it to your regular Park ticket and not the special event ticket.
As you know, guests will be allowed to "park hop" between Disneyland and Disney California Adventure for the first time this year!
Disney California Adventure will remain open to regular guests, which is why you can add MaxPass to your regular Park ticket since all operations will stay the same through Park closing.
I hope this helps.
Thanks for visiting the Disney Parks Moms Panel!
Have a freakishly fun time!
-Anabel M.