Can I add a few seconds before my first line?

Yes.

If the first line belongs to another character and you want a moment before it starts:

1. Tap the three dots ⋯ beside that line.
2. Tap Add Beat Above.
3. Use the slider to set the length (1 – 10 seconds) → Add.
4. To adjust later, tap the three dots ⋯ on the beat → Edit Beat Length.