I haven't found a way to do this in Sass.
My workaround is to import third part libraries directly from their URLs. Of course this only works for CSS libraries that are served via URLs. It's still importing multiple files but at least I don't have to rename files and otherwise manage a modified copy of the vendor's CSS library.
Example:
// ORIGINAL: locally managed, modified copy (bad), no @import in output (good)
@import 'my_modified_copy/closure/goog/css/common.scss';
// WORKAROUND: vendor-managed (good): @import in output (bad but acceptable)
@import 'http://closure-library.googlecode.com/svn/trunk/closure/goog/css/tab.css';
Before release, I'll probably add a script to pull a current version from the vendor and rename all .css files as .scss files. But for now, the above workaround is acceptable for development.