The Line Items area is just too complicated to allow online customization.  In general, any custom field you add to either Quotes, Orders or Invoices must be added to the others due to the ability to convert a Quote to an Order and an Order to an Invoice.

You have to manually add the custom field to the control EditLineItemsView.ascx, add add the custom field to the EdtiView.ascx files for Quotes, Orders and Invoices.