I wish to simply divide a minim rest in a 4/4 time into two crotchet rests in order to have a fermata on the final crotchet rest. Does anyone know if this is possible? 
Yes you can input rests, you’d just need to make sure Force Duration is activated (press O, or click the clamp icon in the Notes toolbox on the left) - this inputs explicit rests with your specified duration.
(Dorico automatically ‘groups’ rests together when they’re implicit rests, as this allows you to think only about the notes you want most of the time - more info about implicit rests here.)