Recording an introduction

How can I record an introduction to audio I’ve already imported and edited?

One way would be to mute the current track, record the new bit, unmute the first track, drag the first track to the right with the Time Shift tool

Every time you press Record from Stop, Audacity will produce a fresh audio track below the existing one[s]. If you mute the existing one with the [MUTE] button, it would be just like making a totally fresh recording.