Keyboard with wl-500gP possible?

[dyonisos]

Is it possible to connect a keyboard to the wl-500g Premium and if yes which interface (serial adapter?) is necessary?

[MMCM]

You could connect an ancient serial terminal to one of the serial console ports. Then you would have a screen too ;-)
Modern keyboards either have PS/2 or USB connectors, that leaves USB as a possible solution.
If you have the right kernel modules (don't know which without researching), you could access the keyboard in some way.