You need to add this new property to your list of available properties using the Property Definition Editor - if using a Central Library then from within Library Manager/xDM Library depending on which version of software you're using. If using the netlist flow then from the Tools menu. If you are creating user properties in the netlist flow I suggest you take a copy of the delivered property file and store it in your WDIR (user or corporate depending on your environment). As you have posted under Expedition then most likely you are using a Central Library.
Once the new property is added to the prp file you can add it to your symbols and populate it from DxDataBook. For more info see the on-line help.
Thank you very much for your answer - really quick!