- Add the same download from the same link to the new download manager.
- When the new temporary file has been created, pause the download manager.
- Replace the New Temp file with the Old Temp File.
- Close and restart the download manager
- Resume downloading.
The above method has worked with Free Download Manager.
If it's a torrent you're talking about, insert the following step between step 4 and 5:
Force Recheck (uTorrent Context Menu)