Yes, it is possible.
The process of account creation will be: [Generate master player account] -> [create a title player account according regarding the title ID given].
When master player account is created, there will be identities binding with it. For example, you are using LoginWithCustomId with CreateAccount set as True, the process will be [Generate master player account] -> [create a title player account according regarding the title ID given]-> [Add specified custom ID to this master account as an Identity]. Hence, you may link any other 3-rd party platform accounts as identities to this master player account.
In addition, to link an email address/password to an existing account, the player should firstly login in successfully (with the Google Account you have mentioned), then call client API: AddUsernamePassword.
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