Personalize by adding another accounting segment to Internet Expense General Information

Currently on the General Information Page of Internet Expense, "cost Center" segment is defaulted from employee HR and can be override and the information will default to Expense Allocation page. How do I add another accounting segment of the accounting flexfield structure to the General Information page with default from HR and also default down to Expense Allocation page, i.e. if I change the HR default on the General Information page, the allocation is changed too? I was able to add this extra field on the general page but not able to default the value from HR.
Please tell me how to personalize this extra segment.