OB52 Posting period, creating new field

Hi,

In the transaction OB52 "Posting periods" I have to mention three different periods "current period 1-2/2010", periods for auditors 12-13/2009 and period 1/2011 (for future posting). For the two periods I have two fields to do so. How can I enter the third period i.e. how can I create a new field?

Thanks