Add USB-C Jack to USB Device
Formally the task of adapting an old USB Type-B device into Type-C environment is solved by using a Type-B to Type-C "legacy cable assembly". This is an official way, as per USB specifications. All it takes is a single cable, and the Type-C end will have all necessary attributes of representing a Type-C device.

However, you want to introduce yet another adapter, Type-B plug to Type-C receptacle. And then to use another, standard C-C cable. It certainly can be accomplished, by wiring a Type-C receptacle to a USB Type-B plug, and having just two pull-down 5.1k resistors on CC1 and CC2 pins.

There are two problems with this approach:


This adapter+cable arrangement introduces yet another connector, and every connector usually introduces imperfections into USB transmission line. For MIDI or Audio devices (which typically are full-speed devices, slow 12Mbps), this is likely not an issue, but for HS devices it might cause flaky behavior.
Likely most important obstacle in the adapter approach is that the USB industry didn't see a need in Type-B plug for attachment to a PCB. All produced Type-B plugs are designed for cables. It is very inconvenient to attach these standard Type-B plugs to any PCB to make a good adapter. 


I would guess this is why no one is making the "B-plug-to-C-receptacle" adapters in industrial quantities.