Hi,
for technical reasons you can't set up a 13th (or more) periods, but you can separate your adjustments with reason codes. The way to go would be to set up a general journal batch with the reason code, so all postings being entered there will have this reason code. For reporting it would be good to add reason code as filter for the chart of accounts, and account schedule (it also works on standard reports then). Unfortunately, Reason Code is not set up as a filter by default. This would have to be done by an (inhouse) NAV developer, but it's fairly easy to achieve.
I would suggest this as best practice :)
with best regards
Jens