Inserting lines in text file

I am already able to replace a certain line number in a text file (thanks to Afrow). However, I am looking around on how to insert a certain line on a text file. I want to be able to goto to a line number and insert a line there making all the lines below it go one line down.

Example:
---before---
line1
line2
line3

---after---
line1
insertedline
line2
line3