Is it possible to extend the fundo 48 X 60 with the 24 X 48 extension kit. Will the slope be maintained or would I need to raise the extension base?

Yes, the wedi shower base extensions work with both the wedi fundo riolito and fundo primo shower bases and extend the slope so no build up is needed. The 24” x 48” extension is sloped 1/4” / ft along the 24” length.