How can I add a natural gas line to an existing brick fireplace?

I will have to say this, since this is a gas line, if you have to ask how to do it, you should not do it yourself.

Call a professional and pay them to do it.

If it is an all brick fireplace, you will need figure the best place to tee off an existing gas line and then hammer drill a hole into the fireplace. If it has a metal liner, you may need to switch to a metal cutting bit to go through it. If you use anything but black iron pipe, make sure your gas company approves it.

Are you buying a commercial burner that has safety controls?