You have two separate reservations, therefore two separate packages.
In order to add on the MYW Dining Plan, you
must purchase a package.
The plan is tied to the number of Resort nights, not the number of ticket days.
Say you're staying at Pop Century for four nights, then moving to AKL for four nights.
You purchase a package consisting of: Four nights Pop; four night MYW dining per room Guest; and ten day park hopper passes (
best buy).
You purchase another package consisting of: Four nights AKL; four night MYW dining per room Guest; and one-day base ticket - which you hold onto and upgrade during your next visit.
Why Disney can't just charge you for the dining plan is that then you're not purchasing a package - THE prerequisite for eligibility for the MYW dining plan.