Modifying genrated pivot table structure

Hello,
Can anyone help me to modify the generated Pivot Table template from BI Publisher Desktop ?

Lets' suppose i have a simple xml like :


<DATA>
<DETAILS>
<PERIOD>JAN-2010</PERIOD>
<ACCOUNT>4111</ACCOUNT>
<SUM>1200</SUM>
<OPEN_PER_BAL>0</OPEN_PER_BAL>
</DETAILS>
<DETAILS>
<PERIOD>JAN-2010</PERIOD>
<ACCOUNT>4112</ACCOUNT>
<SUM>100</SUM>
<OPEN_PER_BAL>0</OPEN_PER_BAL>
</DETAILS>
<DETAILS>
<PERIOD>FEB-2010</PERIOD>
<ACCOUNT>4111</ACCOUNT>
<SUM>1300</SUM>
<OPEN_PER_BAL>10</OPEN_PER_BAL>
</DETAILS>
<DETAILS>
<PERIOD>FEB-2010</PERIOD>
<ACCOUNT>4112</ACCOUNT>
<SUM>102</SUM>
<OPEN_PER_BAL>10</OPEN_PER_BAL>
</DETAILS>
</DATA>

I can generate easily a structure with period as rows, account as columns, and sum like values. But how can i display OPEN_PER_BAL for each month at the end of table ?