Я получил очень умное обходное решение André , работающее в Objective-C с использованием SDK Firebase iOS:
NSString *plistPath = [[NSBundle mainBundle] pathForResource:@"GoogleService-Info" ofType:@"plist"];
FIROptions *secondaryAppOptions = [[FIROptions alloc] initWithContentsOfFile:plistPath];
[FIRApp configureWithName:@"Secondary" options:secondaryAppOptions];
FIRApp *secondaryApp = [FIRApp appNamed:@"Secondary"];
FIRAuth *secondaryAppAuth = [FIRAuth authWithApp:secondaryApp];
[secondaryAppAuth createUserWithEmail:user.email
password:user.password
completion:^(FIRUser * _Nullable user, NSError * _Nullable error) {
[secondaryAppAuth signOut:nil];
}];