The next time you want to add a word which has no transcription in your shadow dictionary (like "geöffnet") switch back to the simon main window (you can keep the add word wizard open of course). Select "Wordlist" from the menu and check the checkbox "Zeige auch unverwendete Wörter an (Schattenlexikon)". You will see all the words of the BOMP dictionary (they are marked red because they have no trainings data associated to them).
Using the filter edit field we can quickly look for "öffnet". We get the transcription: "oe f n @ t".
Now look for "ge" and you will see a lot of words containing "ge". The first hit would be "abartige" with the transcription "a p a: r t I g @". As the SAMPA is very easy to read we can quickly see that "ge" (which sounds the same in "abartige" and "geöffnet" so the SAMPA is also the same) of the word "abartige" is transcribed as "g @".
All that is left to do is put those two parts together and we get: "g @ oe f n @ t"