You may import audio files at any point when making your podcast. However, it is best to import them after you are done recording. If you have any audio tracks already imported before you record, unless you mute them when you start recording, they'll automatically start playing as you record and will become part of the recording itself, meaning you won't be able to edit them separate from the recording anymore. Even if you make changes to the individual tracks, these won't affect what was recorded as part of the recorded audio file.