is there a way to use an existing line symbol in a line style for formatting the outline of a polygon symbol, similar to ArcGIS Desktop's Symbol Selector? Otherwise, complex line symbols would have to be created again.
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.
You can add your support for this functionality over on the ArcGIS Ideas site Having built-in polygon outline styles in ArcGIS Pro
Retrieving data ...