The case of the sheevaplug can be opened by removing the 4 rubber feet, beneath the feet is a screw with a Philips head. I took the old supply out and took it apart to see what happened.
Luckily my uncle owns a electronic shop nearby, www.grootaers.nl He did have a module to build in to fit my needs / measures but he did have an external unit.
Step 1: What to Do
Start by removing the original wiring from the original unit (see below). On the right should be a cable which I already removed.
Basicly I've just cut off the head of the external new unit, stripped the wires and connected them to the wires of the original unit.
I've soldered them together and put a big layer of glue over them so they won't short-circuit.
Step 2: All Done!
Time to close it up and put it back in the corner so nobody will see your ugly but cheap solution!