RWY POSITION SHIFT

Hi,How do I enter a runway position shift into the FMC during the preflight cockpit preparation. For example, at LFPG, I don't want to use the entire length of the rwy 26R due to light weight of the aircraft, but I want (or the ATC tells me) to take off from taxiway W9, and the FMC needs to recalculate all reference speeds for the new rwy length.I didn't see anything about that in the manuals. So I suppose it may not be implemented yet ! In this case, it's something to put in the "to do" list for the next update maybe.rgds / Stephane