As far as I know, you can only add customAttributes to line items when the product is added to the cart. You cannot add them at any other point in the process, or edit them at any point in the future. The original poster on this link shares an exampl...

As far as I can tell, there is no way at all to edit the line item properties once an order has been placed. 

This is for existing orders. The line item has not been fulfilled. The credit card payment has been authorized but not captured.We use the line item properties to store certain custom information, and we sometimes need to change that custom informati...

Do you know how to make that workaround work? I haven’t been able to figure it out but it would work for our situation. 

Did you ever find an answer to this question?

Did you ever find a solution? I am sure one must exist since there are third party apps that say they can edit the line item properties (they seem to remove the existing product and add it back with the new customAttributes).

That guide doesn't say anything about adding or editing the Line Item Properties, which seem to be customAttributes of the line item. Do you have any resources that show how to add a product with customAttributes?

Did you ever find a solution to this? I am also searching for the same question