Hi, Connie! You can definitely book your hotel and theme park passes now, then add your dining plan later if you choose.
Advanced dining reservations, whether you have a dining plan or not, can be added 180 days prior to the date that you want to dine. If you are a Walt Disney World resort guest, you can make your reservations for 180 days plus 10 days, so you can call on the first day your reservation window opens up and make your reservations for up to 10 days of your vacation.
You do not "need" to make dining reservations. I will caution you though that many of the Walt Disney World dining experiences are very popular, and some experiences can fill completely within a couple of days - or even hours - of the 180 day window opening up. If you are hoping to enjoy a character dining experience, or if you want to dine in one of the more popular restaurants (which include Le Cellier at EPCOT; California Grill at the Contemporary Resort, and 'Ohana at the Polynesian Resort), it would be almost necessary to make advanced reservations to make certain you have the experiences that you'd like.
The hardest dining reservation to secure is Cinderella's Royal Table. Breakfast at Cinderella Castle can be filled almost as soon as the reservation center opens up. If you are hoping to dine with the princesses at the Castle, this is a must do reservation.
I hope that helps with your planning!