If I wanted to do this job,I'd solder the elbow onto a short mandrel sticking out of a lathe chuck . The mandrel would be a slip fit inside the hole of the copper. Then,I'd turn down the outside and unsolder it. The same might could be done by supergluing the fitting onto the mandrel,and heating it loose.
If you ONLY have a mill to do this awkward job,use a lathe tool sticking sideways out of the milling vise to turn down a piece of steel held in the spindle. Move the spindle down past the lathe tool to turn it to the slip fit diameter,or crank the knee,if you have a knee mill. Then,use the same method to turn down the copper.
Copper is terrible stuff to turn. Snarly,snaggly,generally obnoxious. Super glue may not be enough,and you may have to solder the fitting on,whichever works. Make the mandrel short as possible and take light cuts,lest the copper snag and bend the mandrel,ruining all.
A whole lot of things are doable as long as you can figure out how to configure the job. You have power,rotation,rigidity just as in a lathe.
