I think they could do that, they've done it on multiple occasions if it matches the pack theme like:I've actually dreamed of the DLC, I dreamed it was part legacy pack. Hopefully my dream comes true.
They gave her the princess ninja outfit as her first Halloween costume
They gave her Kasumi's black school uniform recolor in the newcomer pack.
They gave her the halter top with suspenders and that miniskirt in her intimate DLC in 5U, they just made it more sexual by making the sides sheer cut outs and they made it a zip up that shows underboob.
I think they should use that method since it'll allow them to sneak in one or two legacy costumes without having to do a full pack that won't sell much in Japan.