So I’m back on this matter (I and thus give enough time for a good samartan to help), and now, I think that PlayDTMF is not designed for what I want to do :
I wanted to simulate pressed keys in order to give order to Asterisk (an attented xfert) ; e.g. generate a #2 (it is my combination to performe an attended transfer) via Asterisk on the agent channel, and hope that Asterisk will deal this generation as order to himself (like those keys was indeed pressed by the agent). But I think that those DTMF simulation are intented to the guy at the end of the channel, and not asterisk.