You would extend the copper with a soldered coupling and solder on a male adapter for the tub spout.
There is no sort cut there.
Or, pick up a slip spout. That would be the short cut.
I'm having some issues replacing a tub spout. I purchased a Moen kit, but the spouts their kits come with are only designed to work with a threaded copper pipe. The pipe coming out of the wall is 3" of bare copper, no threads. I'm told this requires a "slip-fit" spout, but I'm having a very difficult time finding one of those in the finish we've chosen. Moen has already told me they don't make one.
Is there any way to get the spout we've already purchased to work on this bare pipe? Moen suggested something called a Nibco fitting. I'm assuming that will require soldering. Also, measuring the inside of the spout there's about 4 7/8" from the edge of the spout (that would meet the wall) to the beginning of the threads, so again I assume some sort of extension will be needed. Will this require a professional plumber to solder on an extension?
I never would have guessed that replacing a tub spout would be so painful.
The only Moen spouts I have seen lately have all been "slip fit", so I suppose that is what the installer expected if he did not have the spout when the valve was installed. Where is the thread on the spout. If it is at the wall, then you need a flush solder adapter which is NOT a diy job to install. If it is at the front of the spout then a male solder adapter will fix you up. There are other ways to do it without soldering but they will cause great distress if you have to remove them, and destroy the spout in the process.
Licensed residential and commercial plumber