Forms Data Controls :: Create Texbox In Datatable - Without Using Gridview Itemtemplate?

i have gridview resulted from sql query as below:

customer_id
items
123
pen
123
paper
456
pen
456
paper
678
pen

then i pivot the gridview by using the datatable.. i call the sql query to the datatable, pivot it using pivoted function and the datatable then become the datasource bind to the gridview..the result of pivoted datatable in gridview as below:

customer_id
pen
paper
123
(empty)
(empty)
456
(empty)
(empty)
678
(empty)
(empty)

my question is..how can i replace the empty column with textbox to be display within the gridview??? i need to use textbox so that user may enter the value of the item within the textbox.. i try using the itemtemplate for gridview but it's doesn't work and the itemtemplate create new column with textbox beside the paper column.. how to input textbox within the datatable in my case??