Broadcast HR from my fenix to the Wahoo elemnt works perfect if the internal optical sensors from the fenix being used. Can I also broadcast the HR signal if a HR- strap is connected to the fenix and if so what to do?
Your chest strap HRM would already be broadcasting your heart rate data, so what would be the point of using the fenix 5X to re-broadcast it, even if such a thing were possible?
Remember, the fenix 5X broadcasts heart rate data over ANT+ and not Bluetooth. Unlike with Bluetooth, multiple compatible devices can receive and process the data being broadcast by a single source using the Heart Rate Monitor profile for ANT+ devices, so you should be able to see identical HR data being broadcast from (the single source of) your ANT+ chest strap HRM concurrently on the fenix 5X and the Wahoo ELEMNT.
If you only have a Bluetooth chest strap HRM, then the most technically appropriate solution for your use case is to get an ANT+ chest strap HRM.