Is there a way,
- with a small board add-on,
- a simple cable incorporating some specialized chip,
to add a usb slave port to a RPi, to let it be used as a device?
Even though the USB port of the RPi is technically an On-the-go (OTG) chip that should support both a reduced set of host and client functionality the B/B+ type of the RPi does not support the device mode. That is related to the included USB hub and the fact that the ethernet is tunneled through USB (see).
If everything else fails, there are examples of software based USB stacks to emulate an USB device on a microcontroller (e.g. the Igor Plug an IR remote control receiver based on an Atmel microcontroller). Of course such attempts only adress low-bandwith features such as human-interface devices (HIDs). If one desperately needs both USB host and device on a board with RPi capabilities one should also look into the BeagleBlack Bone boards.