Thank you borg!
photron, while learning more about M-Bus I think that the quickest way for me to build something that works is to use an off-the-shelf Modbus to M-Bus interface, so I bought a Hiquel converter. It's based on a Texas Instrument MSP430F1611 mixed signal micro controller. By using a USB to RS485 interface I have already configured this converter to read values off my Kamstrup 401 meter and I can display them with the provided configuration software.
Now I want to connect the converter to my existing Tinkerforge Modbus setup. I have been looking at your APIs, is there a way to issue Modbus commands (at application layer level) in order to read the registers of the converter or is Modbus communication achieved in the firmware of your master bricks?