New light and light switch problem

Hi
I am installing a second light in a room, but the only power source is a switch loop. So my question is - can I take power of the existing light fixture to create a new power source on which I can then install the new fixture and switch?
Thanks

Of course you can. The real question is SHOULD you?

You'd probably be fine with a single light socket, or if you are adding several CFLs... something low wattage. But you don't really KNOW what else is on that circuit.

If you do know of course, a simple wattage calculation can tell you how much else you can add safely (given the wiring, and breaker, and such).

Good luck!