adding an existing item to an existing product is currently not supported oob as a Web UI specific operation.
Do you have this use case a lot? Would you mind explaining the background on why this is happening for you?
I wonder if a customizing could help here but would appreciate the background of the requirement as well.
our article processes start with the import from SAP. This import creates the new article.
First step of the process is that the product manager assigns the article(s) to the corresponding product - if there is no product, he creates the product.
During the development of a product many articles are created and have to be assigned to a product.
Okay, makes sense. I think it should be possible to contribute a smaller customization for a custom Action menu entry like "Assign to product" with a pop up until this becomes an out of the box feature.