Could I use the 36 .in X 60 .in wedi fundo primo shower kit, along with this 48 .in X 24 .in wedi shower base extension, to create a 36 .in X 84 .in shower base?

Yes, you would just cut 12 .in off the 48 .in side. You still want to maintain the lip at the edge, checkout the video at the bottom of the product page. These extensions are sloped but cutting the longer side to fit still maintains the slope in the right direction.