Copper to pex transition fittings are readily available. Most solder onto the copper, then have the appropriate fitting to either crimp or otherwise attach the pex to it. There are some push-on fittings, but you're better off with the transition ones. For temporary capping things off, you might want to pick up a couple Sharkbite caps. They'll seal things when you get to a stopping point, but can be removed fairly easily.