Pro doesn't support outlines on multipatches for a variety of reasons.
Pro does support outlines on extruded polygons. If you have polys of the building footprint or floor profiles, you can use extrusion to "make them 3D" and use a polygon fill that has an outline, and it will be honored. We have also made the outlines much clearer by using a different rendering technique for them in Pro 1.2 (out soon).