How can I rewire a three-wire ceiling fan installation where there is a two-wire set up?
pull the third wire (same gauge) through the same conduit from the fan to the switch. It possible that the conduit isn't large enough for another wire and you'll need to upgrade it. Use either red or black but make sure you can distinguish the 2 lives (like if you end up with 2 blacks you label that wire at both ends).

You then either need a second switch so one controls the light and the other the fan. Or if it is pullcord operated then the new wire will need to always live; just hook it up before the switch.