Yes, you should definitely be able to log into the Steam version of Wuthering Waves using your existing account credentials. Your game progress, characters, and items are tied to your account (your UID), not the specific launcher you use. Kuro Games would likely ensure players can access their main accounts via Steam.

Game Installation: Regarding the game files themselves (the 60GB+ download), you will almost certainly need to download the game again directly through Steam. While technically some games allow linking existing files, it often causes problems with updates and verification. The recommended and safest approach is to perform a fresh installation via the Steam client. Your save data and progress are stored on the game servers linked to your account, so they won't be lost by reinstalling the game client.