What do you try to import? The XML from an older OJS version? OJS 3.0 has a different XML schema and the old OJS 2.4.x schema is not supported any more.
Where do you want to put a banner in the header? I think, in OJS 3.0 you can only upload logo image in the header, under Settings > Website > Appearance.
Yes, the PKP banner cannot be removed. The only think you could do is to style it with CSS e.g. with “display:none” or so.