When saving, all line endings will be written with whatever the current line ending is set to: files with mixed line endings will be opened ok, but the line endings are always normalized in memory when the file is opened, so mixed line endings will never be saved.
If I may ask, do you actually have a desire for a file with mixed line endings, or is it a case that you’ve got a file with LF line endings, but with a CR character or two inside that is just supposed to be a CR character, and not a line ending as such?