Attaching a 3 mm fiber optic cable to a WS2812 LED
Aside from a light pipe, a hard plastic version of fiber optic cable that is used in most commercial products to redirect light from a surface mount led



you can use a clear epoxy to mate the fiber optic cable to the 5050 package. You must make sure you use enough to create a structurally strong connection. This is why your other attempts fail, as hot glue and super glue will crack as you put lateral force on the 90 degree connection. This won't be easy as you have to hold the cable in place as the epoxy cures. This would probably require a jig of some sort.

An alternative, is to use a 5mm version of the WS2812 led



and either heat shrink the fiber optic cable onto it



or drill a hole and slide the cable in (instructions (and pictures) from makezine).