perl -pi.bak -we's/\z/Your new line\n/ if $. == 2;' your_text_file.txt
With this command line you can insert a new line anywhere you want in a text without overwriting what's in it.
-p causes perl to assume a loop around your file so that you can process it.
i.bak creates a back up file
-w turns on warnings.
-e is the script you want to execute.