There is no way to enter anything in Scoutbook that was done prior to the registration date showing in the national system.
I have had a few instances where paperwork was given to a DE who is no longer with the program and was lost and had to be filled out again. We looked at things like Scout rank requirements, knots, fire building, cooking, etc. that the scout did before the registration became official and entered those items dated after the scout became official. Don't penalize the scout for an adult's shortcomings.