All new enhancements are being done only to Partnership model. From what I remember we no longer do any feature enhancement to Legacy model.
That being said, what is being spoken is affiliate domain.
I am pretty sure that any new features that are being added are not available via legacy model and affiliate domain. There is no option via WAM UI.
The only CA Supported option is to migrate to Partnership model.
I would recommend to use Partnership model. I would not recommend modify the object on the XPS layer (via XPSExplorer). If you do choose to do that and if it works. Please test it. As it would not be something that we would encourage as a solution nor would it be something that we'd have certified.