As far as I understand you want the MKRZero to behave like a PC when you plug in another Arduino board. For example,
Arduino Uno+16u2 <-> USB OTG host cable <-> MKR Zero
The Uno+16u2 presents itself as a USB CDC ACM serial device. But the MKR Zero in USB host mode does not have a driver for this. You could write a driver but it is a lot of work and there are no docs except for the source code.
If you do not want to write a driver, there are two options.
- USB Host Shield Library for the MAX3421E controller
- Teensy 3.6 USB Host
I have not used USB CDC ACM on either one but have heard they both work. As far as I know, the UHS Library does not work on MKR/samd21 boards but does work on AVR, ESP, Teensy, and Due.
WARNING: Clone boards do not always use the 16u2 chip so may require a different driver.