You should extend the wiring from the light, not the switch. There should be two wires on your light fixture, one grounded (neutral), and one ungrounded (hot) conductor that's controlled by the switch. Extend those wires to the fan, along with a grounding conductor. That will allow the switch to control both the light, and the fan simultaneously.

Conduit is a fine way to do it. One of the luxuries of working in conduit is you can install any wires the circuit requires. This is safer too, because it assures that hots and neutrals actually are together. There's no problem with transitioning between cable and conduit of any kind, so long as you are in locations appropriate to both types.

It's also easier to wire, since you aren't stuck with red and black (or marking white or gray neutrals). You can use any color you please; brown, orange, yellow, blue, purple or pink. Even my local lumberyard sells several of those by the foot. Of course green can never be anything but ground, even in a conduit. (thanks gregmac for reminding me to specify what I assume you know, but the next reader might not.)

In metal conduit you must bond the conduit to ground, but you also can let the conduit be the ground, so one less wire to pull. To make a transition from wired ground to conduit ground, the boxes have a hole tapped for 10-32 for attaching the ground wire. For convenience, they sell green screws 10 for a buck, and pre-made ground pigtails 10 for $3.