There's a mountain of people out there that would love to be able to convert Microsoft Publisher files to InDesign. Why don't you get the jump on them and a massive audience by making Affinity Publisher able to open or convert those .pub files on Mac. I think the only way to do it at the moment is to save the .pub files as PDFs them open them. Doing it this way nearly all of the subtle editing features are lost but at least you can copy and paste the basic elements. It would be great just to be able to open them in Publisher too.