Bill HJ is 100% correct IF you could find this kind of tool (swedge) the wall of the copper would split. What can be accomplished in case of emergency that you cannot locate a proper reducing coupling is possibly putting in the smaller tubing into the larger sized one the flattening the over lap and brazing this contraption.
You will have major proplems with the flow including possible erosion due to the rapid in change of velocity (FPS)
I would strongly suggest you contact the Copper Development association Or ask about a TEE DRILL so you can tap into the pipe directly much better job.