The reason there isn't an official SoftwareSerial library for the samd architecture of your MKR Zero and MKR GSM 1400 boards is because these microcontrollers provide superior options.
The first thing to know is that there is a dedicated hardware serial port on pins 13 and 14 named Serial1. This is separate from the Serial interface you use to communicate with the computer over the USB cable. So if you only need one serial port, you can just use this port as usual:
If you need more than one serial port, then you can use the microcontroller's SERCOM capability, as explained in this tutorial: