No personal experience, but I have heard that while this is not what the sodastream is for (instructions say to ONLY carbonate water), it can be done with success.
The method is as follows: get the beer/cider into the soda stream bottle. Get as cold as possible (like 32*F). Attach to Soda Stream. Use ONE 2-second pull of the lever (as opposed to multiple short pulls as with water). Let it sit for 30-90 seconds. Carefully remove, and be ready to reattach if it starts foaming (at least the mess will be in your sodastream CO2 nozzle and not throughout your kitchen. Pour and enjoy. Throughout the process, be prepared to ruin the soda stream and/or disassemble if need be.
Not exactly a precise way to carbonate, and the CO2 bubbles do not hydrolyze (which DOES happen if you carb slowly in a Corny keg and create a finer mouthfeel), but could be done in a pinch.