Entering account for non-capitalizable expenditure items

We have a question for expenditure items from Payables.

We use only one expenditure type for supplier invoices, because the employees that enter the invoice do not have the knowledge to choose how the expenditure is accounted.

The controller reviews the project expenditures and is able to change the flag capitalizable yes/no. Is the expenditure item is set to non-capitalizable, we need te enter a cost account.

I try to set-up a flexfield on expenditure items to enter the cost account. However the form Expenditure Inquiry does not allow to update a flexfield.

Does anyone has an idea to implement this requirement?