I'm not exactly the biggest Scrum fan, but just a few weeks ago, I had this exact issue (we're dependent on getting fixes and changes accepted upstream), and it

I'm not exactly the biggest Scrum fan, but just a few weeks ago, I had this exact issue (we're dependent on getting fixes and changes accepted upstream), and it wasn't a big deal to fit it into the framework. You can only get folks to commit stuff they have responsibility over. External dependencies are not really part of Scrum per se; they are part of the the larger project management part of managing every team. We assigned an LOE task to me to shepherd/beg/nag my bits through. I (nor anyone else dealing with an outside party) couldn't commit to getting it done, but I can commit to helping it along.