Generally, you can’t import custom workouts based on cadence and can’t input cadence requirements through Zwift’s Custom Workout Builder.
However, even though we’re not at all officially affiliated, ZWO Factory does allow you to add workout blocks and change cadence requirements. The site should do all of the conversion for you changing cadence targets to power output targets.
If you can import a created workout with cadence targets, try to test it out without ERG mode activated first. ERG creates a power output target based on speed and cadence, so it likely won’t allow you to target cadence specifically.
Once you have a chance to test out a custom workout, please let me know if it works or not. If it does, zwofactory may become my new favorite third-party site.