I got it …
I keep my file based mqtt thing and channel definition but at the item level I can add my w2kw.js and it then converts the data …
But this means that something does not work when defining the item transformation on file level …
I know that file based configuration is old-fashioned but I can see in human readable format what it does 
So can I.
UID: mqtt:topic:mosquitto:cerberos_sensor_reporter
label: cerberos sensor_reporter
thingTypeUID: mqtt:topic
configuration:
payloadNotAvailable: OFFLINE
availabilityTopic: sensor_reporter/cerberos/status
payloadAvailable: ONLINE
bridgeUID: mqtt:broker:broker
location: Garage
channels:
- id: garagedoor1
channelTypeUID: mqtt:contact
label: Large garage door
description: Large garage door open status
configuration:
stateTopic: sensor_reporter/cerberos/garagedoor1/state
off: CLOSED
on: OPEN
- id: garagedoor2
channelTypeUID: mqtt:contact
label: Small garage door
description: Small garage door open status
configuration:
stateTopic: sensor_reporter/cerberos/garagedoor2/state
off: CLOSED
on: OPEN
- id: garagedoor1_opener
channelTypeUID: mqtt:switch
label: Large garage door opener
description: ""
configuration:
commandTopic: sensor_reporter/cerberos/garagedoor1/cmd
off: OFF
on: ON
- id: garagedoor2_opener
channelTypeUID: mqtt:switch
label: Small garage door opener
description: Small garage door opener controller
configuration:
commandTopic: sensor_reporter/cerberos/garagedoor2/cmd
off: OFF
on: ON
- id: online
channelTypeUID: mqtt:switch
label: Online status
description: Indicates online status of this sensor_reporter
configuration:
stateTopic: sensor_reporter/cerberos/status
off: OFFLINE
on: ONLINE
That’s what the code tab is for.
Yes, I saw that … Maybe I think about switching now … up to now it worked as expected. The transformation seems not be working correctly … maybe a hint to move forward
THANK YOU for supporting me !!!