For those of you that might be looking into the same challenge:

After a very long time, I have finaly found the solution to import the json string into the Fibaro system.
Each value need it’s own variable.

--[[
%% autostart
%% properties
%% events
%% globals
124 value
--]]

MyId = fibaro:getSelfId()
fibaro:debug(MyId)

local uRad = Net.FHttp("192.168.60.150", 80)
local res, status, errorCode = uRad:GET("/j")
fibaro:debug("status "..status.."; errorCode ".. errorCode.."; res ".. res)

local jsonTable = json.decode(res)
fibaro:debug(jsonTable)

fibaro:setGlobal("uRad_temperature", jsonTable.data.temperature) -- Sensor 1: Temperature
fibaro:setGlobal("uRad_humidity", jsonTable.data.humidity) -- Sensor 2: Humidity
fibaro:setGlobal("uRad_pressure", jsonTable.data.pressure) -- Sensor 3: Pressure
fibaro:setGlobal("uRad_voc", jsonTable.data.voc) -- Sensor 4: VOC (Volatile Organic Compounds)
fibaro:setGlobal("uRad_pm25", jsonTable.data.pm25) -- Sensor 5: PM2.5 (Particulate matter)
fibaro:setGlobal("uRad_ch2o", jsonTable.data.ch2o) -- Sensor 6: Formaldehyde
fibaro:setGlobal("uRad_co2", jsonTable.data.co2) -- Sensor 7: Carbon Dioxide
fibaro:setGlobal("uRad_cpm", jsonTable.data.cpm) -- Sensor 8: Radiation
fibaro:setGlobal("uRad_noise", jsonTable.data.noise) -- Sensor 9: Noise
fibaro:sleep(60*1000)

Tho, will still need to figure out how to make my virtual device work as intended, but this is the solution to import the json string.

I think the cost of the A3 is above what most people feel they can afford, so I guess most will choose the lowest cost option.

Could it be an idea to remove the sound sensor from the A3 unit and try to reduce the cost further by maby removing more sensors from it? Then add a connection that let you add an external module to the A3, so it it possible to upgrade it.

With a sound sensor in this external module the problem would be solved, and the entry cost for the A3 would be cheeper.
Tho it would be more costly to buy a complete set, I guess more people would upgrade over time.

Or maby add the sound sensor and connection option to the uRADMonitor model A, and have an upgrade that give what the A3 provides today? I think maby fewer models, but with upgrade option, would be easier to handle in the future.