Nat's method is solid and simple.
An alternative approach would be to first lock the video track, add any audio file to the bin, then set an end time by dragging the blue right-hand bracket (on the timeline below the preview window) to the left until the length of this 'wild' audio clip matches the desired offset time..
Click the chevron to the right of the ADD button (under the preview window) and ADD SEQUENCE AT START.
The 'wild' clip will be inserted ahead of your audio clip, pushing it down the timeline by the offset amount. That clip then can be deleted.