However, Ikea sell extension units, but they are only intended to go on the full height units. This is frustrating as I could fit a four shelf unit into the new office quite easily. Not being an avid DIYer, I considered this and worked out that even I could get the extension units connected to the half height units.
All the needs to happen is for two holes to be drilled in the top of the bookcases. Measuring from the back of the side panels, a hole with a 5mm diameter and 10mm depth (wrap some insulating tape around the drill bit 10mm from the end and stop drilling when it meets the shelf) needs to be drilled with a centre at 241.5mm and at 8.5mm from the edges. Once this has been drilled on either side, Ikea's instructions can be followed again.
One other little refinement I made was flipping over the top shelf of the extension so that the four locking screws are inside the bookcase, rather than being on the top (as a four shelf unit is still low enough to see the top easily).
The extension units have these holes drilled in them so a further extension unit could easily be added to the top making a five shelf unit.
These measurements worked for me today with the units I bought recently. If Ikea modify their design at any point, I cannot guarantee they will work again: if you have found this post while looking to see if the extension units can be added to the 106cm bookcases, do check the measurements carefully on your units.
Update: May 2021
Having just had a loft conversion finished, I'm in the same Ikea/DIY position as I was nearly seven years ago to the day! I have just fitted two extension units which I have just bought to the 106cm Billy shelf units in exactly the same way, using exactly the same measurements.