Edit Existing Dorm
Enter the lot you want to edit and open the cheat screen (control-shift-C) and type in boolprop dormspecifictools disabled false. This will allow you to use the build and buy modes without limitations.
Make a New Dorm
For creating your own dorms from scratch, you need to go add an empty lot to the neighborhood. Decorate the future dorm as you desire (remember to include counter islands, ShinyTyme Cooktop, sprinkler, dorm doors, and dishwasher/sink). Go to the cheat screen and type in changelotzoning dorm. Your lot will now function as a dorm.