It's very unclear what you're trying to ask.
Are you saying you want sprite1's position to be, say, 10 pixels to the right of where it is now? If so, like Jesper said, there's a move() function for that.
Are you saying you want to set sprite1's position to be, say, 10 pixels to the right of and 20 pixels above sprite2? If so, you can already do that quite easily:
sprite1.setPosition(sprite2.getPosition() + sf::Vector2f(10, 20));