The fast forward (FF) command may already be available, but under a different name. Try all the commands in device mode. Else, you might have to learn the command in RAW mode, but you will have to be consistent with the timing since it will ask you to repeat it. It's too bad the original remote does not have a dedicated FF button. This would make learning it in normal mode a lot easier.

Note: Try learning it again but hide the initial IR burst (try this in normal mode first).