A new feature has been added with the release of Whonix 9, that has not had the announcement it deserves yet.
There are currently two ways to set up a Whonix-Custom-Workstation. Either 1) manually create a VM (old, stable way); or 2) download and import Whonix-Custom-Workstation (experimental, testers-only). That latter way is new and testing is welcome!
Documentation on the https://www.whonix.org/wiki/Other_Operating_Systems wiki page has been updated.