Add Attachment to outbound delivery using SDK

Hello

I'm trying to add attachment to an existing outbound delivery document using ABSL Code:

document.AttachmentFolder.CreateFile

I get this error message :

"changing data not possible;data is read-only"

I understand that this is because the document is no more in editable status, but other documents such as supplier invoice / credit notes are updatable regardless or their status.

What can i do to add attachments to this outbound delivery ?

Thanks, Tamir