However, using the insructions here: (with a little modification for UK Echos), then you can do it now.
Once working its actually very good - however, it makes it awfully more difficult for alexa to hear you over the music from sonos. I dont know how the official integration will get round the problem of music from other sources drowning out your voice to Alexa. Clearly when it is playing the music itself, it knows what to expect, so can still hear you over the music (if this makes sense)