The kmail handbook says one can create a new maildir folder and copy messages from an mbox folder into that folder. Will it work in reverse?

All my folders are under “local folders”, and they seem to be in maildir format. Can I make an mbox folder there too? (Made a folder and don’t see how to change format)

As far as I remember you can create a new local folder resource and have it configured in mbox format. I don't think you can do this within an existing local folder resource.

File > New > Add Folder
Folder > Add Folder

I tried to add a couple folders outside "Local Folders", but they spontaneously nested within. I deleted them, and now "Add Folder" is greyed out.
How can I make "Local Mbox Folders"?

In your account settings, you can add a new account of type local folders.