RDL is not possible to convert to pbix because report builder and powerbi dont have the connection its totally different entity but you can use the semantic model inside of powerbi to the report builder as data source or u can fetch your report from published My workspace (apps.powerbi) using premium license you can see the diamond icon purple one and edit on report builder but not vice versa.

I encountered this problem when i am trying to use pagenated reports as i need to get data from my visuals from powerbi and try to convert it using report builder.