The fork cap unscrews from the rod.
The rod is free to move up and down a certain amount and will easily extend beyond the top of the fork.
Once you've unscrewed the cap you should be able to extend the rod.
You will then want to pull out the OEM spacer which is basically a metal cone.
Replace the OEM spacer with a spacer that gives you the preload you want.
Screw the cap back onto the rod (but be careful - at least one guy here ended up stripping the thread as the cap is alluminum and the rod is steel). Torque to specs.
Screw the cap back onto the rod.
PS - To my knowledge you will be the first succesful guy to have done this without removing the forks from the bike.
I assume you know how much preload you want to add. For some info on how other folks did theirs with after market springs do a search with "sonic springs" as the keywords. Note that the sonic spring is a different length from OEM.