I tested it and bought the steam version.

Short version:
There is no way to login with a non-steam account.

Long Version:
The steam version uses 1:1 the same launcher files. The only difference is that steam launches the exe it with a "-IsSteam" argument (https://steamdb.info/app/39210/config/) that can't be removed without some extra tools. If you login its always says that the account has no service account for the platform (steam).

I have no glue why SquareEnix don't let us simple login with any account. So people can buy the starter version on steam if they want. Elders Scrolls Online has many dead license keys because many players does this when the steam version was released.

10€: 7€ to SquareEnix for nothing, 3€ to Steam for 50mb download and steam features. Its a winwin for SE.