it is possible to get an FCM registration token by using the Firebase iOS SDK in the app. You can then use this token to register the device manually, like below:

let deviceDetails = ARTDeviceDetails(id: ULID().ulidString)
deviceDetails.secret = generateSecret()
deviceDetails.platform = "android" //or "ios"
deviceDetails.formFactor = "tablet" //or "phone", "tv", "watch", "desktop", "tablet", "car"
deviceDetails.clientId = "clientA"
deviceDetails.metadata = NSMutableDictionary()
deviceDetails.push.recipient = [
    "transportType": "gcm", //or "apns"
    "registrationToken": "xx"
] { error in
    if let error = error {
    else {

It is important that you save the device details, as at present the local device will default to APNs.