I need to run a coaxial cable wire to an existing building. It cannot be ran overhead, it has to be underground. Instead of direct burial, I already have a conduit with #6 wire in it for power. Can I, or would it interfere with the cable, if I was to run it in the conduit with the power?

Sorry, you can't run a communications cable in the same conduit per the NEC. You'll either need to go with another conduit or direct burial.

Besides, for best results you should keep any coax, datacom, or telecom cable away from power lines.

You say the conduit has a #6 wire in it. Is this the only wire, and is it a ground?

Parallel runs of low voltage cable should be at least 6" away from parallel running power lines. Low and high voltage CANNOT be run in the same conduit.