These threaded connections? If so, I'd screw in a brass nipple, then transition to the copper. The pros may have a better way.
Adding a second panel, especially on a second floor, will probably mean you need to deal with balancing the flow...the lower one will get more heat since it will be harder to push enough flow up another flight. You may need some additional controls/valves to ensure both get enough flow to warm the rooms evenly.