Search your opponent's borders for streets that are next to your border. Once found, place a flag directly next to the street. Click on the flag and click the symbol (green with the red arrows) for connecting the flag to the street. It will not connect if the street was not close enough. Once connected the following tricks may be done. *Note: Never connect this new street system to your own system before disconnecting from your opponent to prevent leaking your resources, people and knights.
Have opponent build on your land
Place a storage hall next to the flag. Connect it with that flag only, but not with your own streets. Your opponent's settlers will eventually begin building the hall on your land. This trick is even more useful if it happens next to a smith. The weapons will be transported into your storage hall, where your knights grow with the opponent's weapons.
Cut the street
Delete your flag.