Broken steel 1/2 inch supply line. Need to replace section that is in a wall cavity.Can the steel pipe be cut and some sort of union or compression fitting be used to connect it to new copper piping without having to thread the existing steel pipe in wall?I do not see a way in wich you could thread the existing pipe in wall cavity due to clearance issues(very tight limited access). This supplyline runs up from basement through kitchen exterior wall to bath on second floor.pipe access is extremely limited.

There is a fitting called a G-coupling, it could be called a different name in different areas. It will form a compression joint onto the steel pipe and then it comes in different configurations on the other end. One would be for 1/2 inch iron pipe threads so you could solder a 1/2 male adaptor onto the copper pipe then the G-coupling would be a compression joint on both the steel pipe as well as the copper.

Compression couplings are not used inside an enclosed walll cavity so I would advise try to back out the galvanized pipe after cutting it with a hack saw or reciprocating saw. I doubt that you'd have room for much else inside a wall.