To my best knowledge there is no way to add a custom workout on Android phone.
I have found though that you can trick the in-game workout creator to show up on Android by using the app full screen in Desktop Mode.
On Samsung phones that feature is called DeX and it is very easy to use. Other OEMs have their own versions too. It is supposed to be a core feature of Android Q, but for the Pixel 3 I don’t know what is the current state of support.
This is the workout editor on a Samsung Galaxy S9 connected to a Dell USB-C monitor.
Else there are community made online zwift workout editors.
And I am currently working on a community driven online .zwo workout runner.