Header text manipulation (VA22)

Hi,

Is there any way we can manipulate header texts in sales documents in runtime, i.e can I fill the header texts through userexits while users use va21 or va22 ?

Complete scenario:

We have not configured text determination because the there are no fixed rules mentioned by our client.

Hence the users are keying in text based on some other characteristics. Once the quotation is final then they need to change the customer number in VA22 which is wiping off the texts. Now I need to fill that text box with the texts keyed in while creating the quotation. I can read the texts using the function module "READ_TEXT" but the question is how can I fill the textarea so that the user can modify the text before saving.