I have a project created by others that includes thousands of class files and has the package names explicitly typed out for every reference to any of their classes. It look
For a single type, eclipse offers the 'Add import' action (Shift+Ctrl+M). It does exactly what you want - with the big, big limitation: you have to place the cursor on a type and it will only affect that 'selected' occurrence.
But maybe this action can be used in a scripted/global method. A JDT plugin could crawl through the AST and call this action on every type it finds.