At Volkszähler you can read data from some of the network operators' meters using a D0 reader. This data can then be made available to the cFos Charging Manager with vzlogger.
vzlogger provides a JSON file with current counter values. An example file can be found here: vzlogger-out.json (as ZIP archive).

To be able to read them with the cFos Charging Manager, you need to create a customised meter definition file and then upload it as a Customised Meter. An example file can be found here: cfos-meter-vzlogger-bi.json (as ZIP archive).
vzlogger creates an element in the array "data" for each counter value. The values are then accessed as tuple[0][1].

The order in which the meter values appear in the JSON file depends on the order of configuration in vzlogger. If you operate a meter with a positive and negative sign of the power for mains supply (positive) and feed-in (negative), you must apply the following patch for vzlogger and compile vzlogger accordingly: https://github.com/volkszaehler/vzlogger/pull/476.

Many thanks to Joachim B. for the explanation and for providing the sample files.