I have an uf 10-2 and would like to run a 30amp 2 pole circuit to my shed, the trench is open, can I add a third conductor in conduit to accomplish this.

First of all, for a sub panel you require 4 wires, 2 hots, a neutral, and a ground. So is it a circuit, or a sub-panel? Circuits have different rules to follow than sub-panels.