Thank you so much guidol.
Seems like quite a simple hack anyone can do!
For the dockstar the power could be inserted via the external plug, but for the
sheeva you have to open the sheeva and remove the power supply and connect the
power to the red/black cables which are on the motherboard (coming from the powersupply).
And if you use the ATX-Power-Supply without a case, then you have to connect the green cable (Pin 14)
for a short time to ground (black) to switch on the power supply.