Since every mobile devices from Apple supporting iOS10
, also support Bluetooth Low Energy, yes, you can send credentials from the phone to a Bluetooth Smart peripheral device, such as a Raspberry Pi equipped with a Bluetooth 4.0 dongle for instance.
See the list of devices supporting iOS 10
See the list of Apple devices supporting Bluetooth 4.0
You simply require a custom service and characteristic on the peripheral, on which your iOS device will write the Wi-Fi credentials to store.
You can further add up security by encrypting the Wi-Fi credentials using a pre-shared key (a unique symetric key per users for instance) before sending it to the Raspberry Pi.