How to override / extend an existing converter/device?

[hinneLinks]

I have an Unitec 30946, according to the manual p. 12 "Temperaturkorrektur" it supports calibration, the current zigbee2mqtt-herdsman-converter does not.

I'd like to try out and make it work, play with the Tuya datapoints etc., but I don't know how.
How can I override / extend an existing converter/device?

So far i tried:
(My zigbee2mqtt installation is inside a docker container)

• Manually alter the siterwell.js inside the container (that got overwritten on restart)
• tried to use the original code (added with some code taken from other similar devices like this and this) as an external converter, both as .js and .ts.
  • I've tried to add the code with zigbee2mqtt/bridge/request/converter/save, but run into issues masking the ", so i've edited the file directly on my server
  • For each try, I've removed my Device from zigbee2mqtt, restarted and re-added the device - i dont know, if my changes did do anything, it seems the original converter was used.
  • Eventually, the external_converter got renamed ".invalid", but I could'nt find a hint in the Logs, why.
  • I've never seen a payload under zigbee2mqtt/bridge/converters using MQTT Explorer