This has popped up one or twice in the Questions forum. With assistance from @DonManfred, I added this for my use. Others may benefit.
For reference: https://www.b4x.com/android/forum/threads/solved-creating-sub-directory-with-externalstorage.109776/
For reference: https://www.b4x.com/android/forum/threads/solved-creating-sub-directory-with-externalstorage.109776/
B4X:
'Creates a new directory
Public Sub FindDirOrCreate(Parent As ExternalFile, Name As String) As ExternalFile
Dim f As ExternalFile = FindFile(Parent, Name)
If f.IsInitialized = False Then
Return DocumentFileToExternalFile(Parent.Native.RunMethod("createDirectory", Array(Name)))
Else
Return f
End If
End Sub