Is it possible to post to a special period using KB21N?

Hello Experts.

I need to post a KB21N transaction in dated 12/31/2010 but in period 13. In KB21N, the period field is grayed out. Is there a way to make this an input field? If not, is there a BAPI or FM that I can use to create an activity allocation and post it to period 13?

I tried BAPI_ACC_ACTIVITY_ALLOC_POST and it did not allow me to post to period 13.

Or is there another transaction which we can post secondary cost elements and move shift cost from an order to another order in period 13? Is KB65 my only option?

Thanks in advance.

Arnold