That's fine to do it.
All you need to update fields in program and add one ! more field in database.
Sales order details table and
debtor trans details
Add a column for delivery date .
And then go to sales/includes/ui
Here sales_order_ui.inc file is there. Just find below functions
add_to_order. - pass one more parameter and do it.
display_order_summary
sales_order_item_controls
And now open sales/includes/cart_class.inc update the line_details class with one more field and for date and also pass it to add_to_order function here.
With in the read and write functions it calls some more db operational functions. Just update that to complete it
Sales order entry.php
Pass the date parameter to add to cart calling function