import glob

# Your custom code.  This is a list of strings.  The commas separate
# the individual strings, and the \ means "continue on the next line"
tylersCode = [\
    "blahblahblah",\
    "my name is Tyler",\
    "and I don't know how to program",\
    "just kidding, haha."]

# The number of lines to skip in each file before inserting your custom code
numberOfLines = 3

# The extension of all the files you want to fix.
extension = '*.txt'

# The filepath folder to look in
folderpath = 'C:\\path\\to\\the\\files\\'


toProcess = glob.glob(folderpath+extension)

for each in toProcess:
    fileRead = open(each)

    lines = []
    for line in fileRead:
        lines.append(line)
    fileRead.close()

    fileWrite = open(each, 'w')
    fileWrite.writelines(lines[:numberOfLines])
    fileWrite.writelines(yourCode)
    fileWrite.writelines(lines[numberOfLines:])
    fileWrite.close()