Currently there is no functional equivalent in ArcGIS Pro to the symbol selector in ArcMap allowing you to use predefined line symbols as a polygon outline. This is because in ArcGIS Pro the polygon outline is a stroke symbol rather than a line symbol. However in the Properties section of the Format Polygon Symbol pane you can add symbol layers and effects to recreate for the polygon outlines, many of the same pre-defined styles for line features.
Still no solution to this? This is infuriating to say the least. Not being able to apply a line style as a polygon outline seems like a colossal oversight to me. Even if the style type has been changed, Pro should be smart enough to convert it as it is essentially the same set of parameters.
This is the type of stuff that not only prevents people from switching to Pro, but hardens people against it's use.