Hi divetoni,
here's a great(?) non answer:
Supposedly ( I got my YS 90 DX in 2002- newer versions may be different) the diffuser can't be used with the fiber optic cable, because the diffuser blocks the sensor (?) in the camera that the cable must be aligned to.
However, I recall, but can't remember where I saw it, a posting (Not on SB, I think. Possibly on "wetpixel" or "digital diver"?) where somebody "jury-rigged" a connection by cutting and gluing parts from the diffuser with parts from the ring that is used to connect the fiber optic cable to the strobe/sensor.
In short-and from a "physical connection only" standpoint- it can be done, but I'm not sure what the details are.
Maybe this will lead to others who can provide details.
Take care,
Mike