Since silence is also a record terminating event, the logic for trimming the trailing silence is not something you can turn off. However I believe it will not trim more than a max terminating event amount off (something like 3 seconds) - if you're recording through the SA you should be able to put more silence than you want on there and it'll only trim the last chunk of it off.
Another approach is to append to the recording with silence WAV files using the media master control - there are several 1 and 2 second long silence WAV files in the prompt set you can use for this and the media master control itself will always just append the WAV file on there, the silence trimming is only done by the phone conversation at the termination of a recording.
The first prompt WAV file that has 2 seconds of silence that you can use can be found at:
\commserver\localize\prompts\enu\g711\AvAddrSearch\AvAddrSearchENU026.wav.
Just tack on as many as you need and that should do the trick.
You can also record the greetings as a stand alone wav file (i.e. using sound recorder or whatever) and import that using the media master control, but in general this wont sound as good as recording it using the phone interface.
