Unfortunately, previous saves and choices cannot be transferred into the Definitive Series. However, in A New Frontier (Season Three) and The Final Season, there is an in-game story builder that allows you to recreate your old choices.
For example, if you have previously played Seasons One and Two on your PS4, you can start the Definitive Series right on A New Frontier (Season Three). You can select to recreate your backstory and choices when you start a new game.
When you finish A New Frontier, your choices will be automatically imported into The Final Season. Any save data you create directly within the Definitive Series can be used only in the Definitive Series.