To add a dining plan, the reservation must be a package, unless it is a DVC reservation.
And yes, a package reservation must be paid in full at least 45 days from check in.
can you book as a room only and then add dining prior to three days ahead of check in? (ap holder and fl resident)
No. You can't add a dining plan to a room only reservation. That reservation would have to be cancelled and rebooked as a package. You could book a ticketless package (room plus dining only) to get around the ticket purchase requirement on a package - but you'd still be subject to the package rules for payment, changes and cancellations, so full payment would still be due at 45 days out.
You could actually book a package within 45 days and then pay it in full as soon as you book it. If that is really what you want to do then there isn't much point in booking the room-only since it won't act as a placeholder, and you can't apply any payment you put toward the room-only to the new package. and if you want FP+ at 60 days, you need to have a reservation at 60 days.
It appears that if you get a reservations CM who knows what they are doing, they can rebook the package in the same resort and room type, but you'd still have whatever you paid on the room-only refunded and need to pay for the new reservation. And then you cancelled your room reservation - what happens to your FP+? I'm still not clear on that one. - ADRs would remain the same though.
The only way to avoid the package requirement entirely is to own DVC or rent DVC points. I'm not entirely sure about cast member reservations, that might be another way depending on what they are willing to offer for cast members.
