It's not the best idea to mix in a spare that's sheathed. Your best option is to replace the entire line, as long as your repair piece is long enough. Make sure to match the length with the other side, not the one that's damaged. If you can't do that, you might be able to splice in a short looped connector, but just ensure that the line you're using for the repair is stronger. It's a bit more complicated, but it can be done