The basis of this conversion is to make two box frames out of stud timber, unscrew the desk top from the frame and then insert the box frames between the desk top and the frame - easy!
Step 1: measure how much you want to raise the desk by. Keep in mind the upper and lower range of the adjusting system. For me the height increase need to be about 350-400mm.
Step 2: Measure and but the wood to match the metal frame. In this case, 600mm. So my cutting list was four lengths of 600x90x35mm (the tops and bottoms of the box frame for both sides) and six lengths of 350x90x35mm (three uprights on each side).
Step 3: Clamp, glue and screw (don't forget your pilot holes!) the frames together using a carpenters triangle to get it all square before screwing it in. Let the glue set before continuing.
Step 4: Screw the box frames on to the metal desk frame, making sure that everything is lined up, then screw the desk top on top of the top section, topping it off. Its tops!
Step 5: Use a small off cut as a reinforcing piece for the adjusting mechanism. (this next bit really should be done at the end, but I'm impatient and am happy with it as-is for now) Re-position the adjusting mechanism to its new home.
Step 6: (awaiting more money) What I really should do now is to make a shelf across the metal frame cross-bar with a plywood back to reinforce everything and then clad the frames in some thin plywood and paint to finish it off, but I'm lazy and it works (and it is actually quite sturdy)!
Possible step 7: In the future I would like to add the ability to angle the desk top, but that involves buying hinges and devising a way of locking it in place...probably using little wooden plugs...maybe a linear actuator?...