A workaround could be:
Buy/DIY a USB-C to minijack adapter, and feed the headphone output (or any other analog audio signal) into the microphone input.
However, I'm unsure if you'll have to change the gain of the analog signal before feeding it back in.
A different solution could include Bluetooth mic input as the pure input source using a BT transmitter with a minijack input, but that will introduce some latency. However, the later versions of the protocol can work with a very low, possibly unnoticeable, latency.