Is it possible you can get meltan in ultramoon or transfer into ultramoon. On the website, it said it's tradeable?
Unfortunately you can't
No. You cannot obtain Meltan in Ultra Sun/Ultra Moon through any method, unless you hack a Pokémon in Ultra Sun/Ultra Moon to look like Meltan. Meltan can be obtained with the games Let's Go! Eevee/Let's Go! Pikachu and higher, though no one knows how yet. I heard that Meltan is a generation 7 Pokémon, though it cannot be obtained in the generation 7 games. "Trade/migrate from another game" means the Pokémon cannot be obtained in that game or it means the Pokémon is an event-exclusive Pokémon or it means PokeMaster never added the locations. Also if a Pokémon is a version-exclusive Pokémon (example: Alolan Vulpix is only available in Ultra Sun, but Alolan Sandsrew is only available in Ultra Moon), then "Trade/migrate from another game" is possible.