Expense Report tracking for multiple employees

We are on R12.1.3.  We use Payables, Projects/Grants, and iExpense (along with other modules).  We have Administrative Assistants (AA) who have credit cards.  They use their credit card to order supplies for the people in their department.  However, we want to be able to track the expense against the person who used the item.  So, the AA orders something using a credit card.  We receive a file from the bank showing all credit card charges that is loaded into AP.  When the AA goes to do an expense report, the credit card charges are associated with the AA.  How can we