Question Acquiring the culling renderers of shadow casters when draw directional shadowmap

I'm working on a custom CSM Scrolling approach of directional shadowmap in URP (http://advances.realtimerendering.com/s2012/insomniac/Acton-CSM_Scrolling(Siggraph2012).pdf). By now I can scroll the shadowmap from one directional light view to another. From the method metioned above I should render addtional stational renderers to fill the margin. However, I find there is no way to get the shadow casters which will be used for rendering shadowmap. Is there any advice to solve the problem?