Yeah, you could probably grab the PKGBUILD from Arch and build it, You might need to make some tweaks and perhaps build some other packages as well. There's nothing stopping you, but why do it?

You can just use regular Arch (or any other distro) and not have the limitations of SteamOS (read only filesystem, old and fewer packages, etc). Install Steam and use Big Picture Mode and you're good.

If you're making a handheld, it's probably worth the effort. If it's a desktop PC, just use a regular desktop Linux distro and add stuff you like from SteamOS.