Michelle,
We figured it out, though it seemed like kind of a round-about way to do so. What we did was logged into our org portal. Then we created and saved 3 blank webmaps to our ArcGIS Online account. One with Bing Maps as basemap, one with Bing Aerials, and one with Bing Hybrid. Once the 3 webmaps were saved, we shared them to the "Bing" Group that we had created in order to get Bing basemaps to work through ArcGIS Online. Once we did that, those 3 maps showed up as basemap options in ArcGIS Pro. Hope this helps...