If you want to open a page with your browser and make it believe you're someone else, then no way.
The only thing I heard of is that mozilla allows custom user-agent, but that's all you can modify in browsers.
If you want to request a page with your hand-written program, say, php script, then you should learn a lot about [man]fsockopen[/man] and http protocol. Then you can send any nformation you want. But it would be a big achievement, to make it work 🙂
I believe there may be some libraries or classes somewhere on the Net for it, try searching.