Configure MIDI Device as Output

Hello guys!

I have a Digital Piano (my MIDI device, a Casio CDP-130) and a MacBook Pro with MacOS Sierra.

My piano have its own speakers (two, right and left), which are the ones I want to be using, instead of my MacBook' speakers.

I'm connecting my MIDI device to my MacBook with a USB cable (Type-B to Type-A), there are no other plugs for I/O in the piano, just for power, pedal and headphone. So, it is both input and output together.

I've already used my piano as MIDI Input to my MacBook with GarageBand, as a VST and recorded some things. Also, in the system's "MIDI Studio" (in the "Audio MIDI Setup") my piano is right there, both for Input and Output, and they are working fine. I've tested it with the function "Enter test MIDI setup mode", when I press and release a key from my piano I can see and hear the system's feedback and when I press the output icon, the piano's speakers make a loud sound, showing it is identified and working fine.

However, in "System Preferences>Sound" window, there is only one option, for both I/O, which is the "Built-in" option ("Internal Speakers" and "Internal Microphone"). Also, in the GarageBand's settings there is only two options for the I/O, the "System Setting" or "Built-in Output/Microphone" (since the system's setting is the built-in, both are the same thing).

I've also searched about how to configure it, but all I could find were tutorials of how to configure MIDI devices when they are not being detected at all by the system or the farther it goes is to the "MIDI Studio" window, to verify if the device is there and not "grayed".

Someone could, please, tell me how can I set my MIDI device as output in this scenario? Also, if possible, not only for GarageBand, for other apps or the system itself as well, as stereo sound.

I thank you in advance for your patience and time.

PS: I'm sorry if it is hard to understand my post. English is not my main language.