There's no 'insertion' functionality for files: you have to create a new file with the appropriate content:
Read line by line from input file (you may use the
StreamReader.ReadLine Method[
^]) and output the read line to the ouput file, until line count reaches 500.
Then write the '10 rows' to the output file and, finally, resume reading line by line the input file (and write every line to the output file) until the end-of-(input)file.