1. Add your list of barcodes, MMSIDs, or whatever you would like joined into a new Notepad++ document
2. Press CTRL+H to open the Replace Box
3. Find what: $
4. Replace with: ;
5. Regular expression checked.
6. Matches new line checked.

1. Press CTRL+A to select all.
2. Press CTRL+J to join lines.

Scroll to the end of the line and remove the final semicolon(s)