It depends do you want them for franchise mode or season mode? If it is franchise id significantly lower the overalls of the real prospects so they don’t have any impact on that mode and then I’d just recreate all the players from the draft except I’d make them 18 years old before assigning them to their teams.
If you just want to play a season with Dahlin, Zadina, Svechinkov etc etc on their teams I have found a way to do this. Simply recreate a team that’s jerseys are easy to create like say New Jersey for example and put all the prospects on that team along with the team’s actual players . You have 40 player slots in create a team so there is more than enough room fit a lot of the 2018 1st rounders in there. After that sub out the NHL team you created with the custom team version of it and then start up your season mode. Make sure you are controlling every team and just simply trade the prospects on your custom NHL team back to their real teams.
It is a bit of a pain in the a**, but it does work for season mode. If you have any questions feel free to ask