[KevinH]

Remember, those folder names in Sigil's BookBrowser are just "virtual folders" where files are grouped by file types. The files in them can actually exist in any place in the epub.

In Sigil you can easily create new folders/bookpaths and move files between real folders using Sigil's interface.

That will not change the virtual grouping name they appear under.

To aid in this, make sure you set Sigil Preferences to show the full bookpath of all files in BookBrowser.

So after creating a bookpath to a new folder, you can bulk select some files and move them to that newly created folder using the Move menu/command.

This is how Restructure to Sigil Norm is done as well.

But to your case ... do not add the files manually to the epub. Add the new folder using BookBrowser, then just import the files normally, and they will be added to the manifest. Then use Move command to move them to where you want.

Once a folder gets enough files of the same type, it should automagically become the default destination folder for all files of that type when importing existing files.