I think you'll probably have a difficult time getting an adapter to work--possibly even a Micro-B to type-C cable.

Here's where it gets annoying.

USB is traditionally directed; you have a host and a client. This was codified in the physical standard until type-C. Type-A ports (and the respective jacks) are intended for hosts while type-B was intended for clients. A usb-C to micro-B cable therefore will be intended to provide power/data from the C side to the B side and in almost every case this will not work in the other direction.

In your case the battery bank is intended as a host to provide power. You might find a Micro-B to C that works, but it'll probably be out of spec.

So why does the Micro-AB port exist? Well the micro part implies your device is mobile. Being mobile might mean you have a battery, and having a battery means you might want to charge it. That means your device can likely operate as a host (to charge your phone) OR a client (to charge your battery) through the same port.

Since type-C is intended to be fully reversible the type-C port and cable will have wiring to facilitate that. However a Micro-B is intended to be directional and with a fixed orientation so the wiring, even from a type-C jack will facilitate that.

I wish I'd remembered that before the false hope, but it completely slipped my mind.

You will probably be able to use a USB-C extender if the only problem is that the original cable is too short. They have some at Walmart for $2 USD.