Will wrote

Which imaging utilities can read either a DOS or NTFS partition and copy the image to a file on an NTFS file system?

If you really meant write instead of the word copy, then any of
the major imagers will do that fine. I prefer Acronis True Image.

The point is I want to have a library of images as large files under NTFS, rather than having to index a large number
of physical drives, each of which would have one or more partitions.

Not clear what the first part of that means.