Do you need to record it, or is it already recorded?
If you need to record it, look into the Voice Over Tool in FCP. You can find out how to set it up and use it in the help menu of FCP. Type 'voice' in the search bar at the top right. Of course, you will need a compatible microphone.
If your voice has already been recorded elsewhere, just put it on the timeline like any other audio. Make sure it's an aiff at 48khz for ease of use.