get-content input.txt|foreach{$_+";"}|out-file output.txt

Now, be warned that PowerShell can play tricks with hidden characters in situations like this...

To replace the content without creating a new file:

(gc file.txt) | % { $_ + ";" } | sc file.txt

thank you very much....

i just noticed that the file has 1 million rows. Marco's solution will be the least resource intensive method. You could then use the following function to end with the same file name as you started with:

function Add-TXT {
param (
[string]$file,
[string]$temp = $env:TEMP + "\output.txt"
)
gc $file | % { $_ + ";" } | out-file $temp

ri $file -Force

$tempFldr = $temp.TrimEnd($temp.substring(($temp.LastIndexOf("\")),(($temp.Length - $temp.LastIndexOf("\") - 1))))
$fileFldr = $file.TrimEnd($file.substring(($file.LastIndexOf("\")),(($file.Length - $file.LastIndexOf("\") - 1))))

if ($tempFldr -eq $fileFldr) {
rni $temp $file
}
else {
mi $temp $file
}
}

Then call it like such in your script:
Add-TXT FileToCopy