Is there a way to use or copy the workouts from a plan for instance Gravel Grinder and use them as a custom workout?
You could sign up for a TrainingPeaks account and transpose the workouts over from whatsonzwift.com.
It’s a long-winded cumbersome way to do it (particularly if you’ve got a free TP account and have to do it EVERY time) but it should work.
WhatsOnZwift details each Gravel Grinder workout: here.
WhatsOnZwift does not have those available for download.
You could use the WhatsOnZwift descriptions to build the workout inside Zwift and save it as a custom workout, or use a third party workout creator such as zwofactory.com or the app.trainingpeaks.com system already mentioned.