Downstream users cannot remove clauses put in place by the copyright holder, even for alterations that they make, because those are derived works. A pure addition that you make to a copylefted program is your copyright; you have to license it in a way which is compatible with that program, but otherwise it can be anything. E.g. you can add 2-Clause-BSD-licensed code to a GPL-ed program and then redistribute that to your own downstream users. The GPL still applies to the thing as a whole; users can use just your portion alone under its BSD license (like borrow the code into another program).