I like the Opus dialog for creating folders.
Although it would be possible to create something similar via scripting, to make things easier you could consider adding a new argument for CREATEFOLDER, SOURCE, that lets you override the source folder where the new folders will be created.
Then, you could call that dialog to create new folders inside the selected folder, like this.
=return "CREATEFOLDER ASK" + (seldirs == 1 ? (" SOURCE=""" + selpath + """"):"")
PS: Maybe a tooltip somewhere to know what the current source is would be great.
Thanks for considering it!