With the file system object, you can't read and write at the same time. So if you want to edit the content of each line in a text file, it's a little tricky, because you first read the current content into a variable, then you write that variable to the same file.
In this example, the goal is to stick a "," at the end of each line.
STEP 1 -- read the data
Dim mydata, f
Set f = fso.OpenTextFile(newfile)
Do Until f.AtEndOfStream
mydata=mydata&rtrim(f.ReadLine) & "," & vbcrlf
Loop
f.close
STEP 2 -- write the data (this will overwrite whatever was previously in the file)/
Set f = fso.OpenTextFile(newfile, 2, true)
f.writeline mydata
f.Close