Is it possible to focus a RGB LED onto a 8mm fiber bundle?
Unfortunately no. Presently we do not have a fiber injector lens that will work with a Tri-LED assembly. This would be a pretty specialized lens and it is unlikely any of the lens manufacturers will introduce one in the future.
We do have a fiber injector lens that will work with a single LED. You will find details here:
http://www.luxeonstar.com/Carclo-Fiber-Coupling-20mm-Lens-p/10356.htm
And we also have a fiber injector lens for our 7 LED assembly here:
http://www.luxeonstar.com/Polymer-Optics-7-Cell-Cluster-Concentrator-Lens-p/263.htm