The online ZWO editor gives you an easy way to create a workout with predefined steps given duration and power target.
It has just been updated such that the power target can be either an absolute number (in watt) or a percentage of FTP (this is the new bit).
A quick update, 8 hours later:
I made it possible to enter a zone number from 1 to 6, too. The format must be like z1, Z3, or simply 6. In general, if the ‘power’ number is less than or equal to 6, it will be interpreted as a zone. See example 2 in the tool.
The zones are the same as Zwift uses.
You can read more about the Simple ZWO Creator and the supported input format in the original announcement post or simply try the built-in examples to see how it works.