This method I describe can be used with **any** bittorrent client.
This is a 2 step process:
1. Add the same .torrent file to the new client
2. As save location choose EXACTLY the same as the old client.
Qbt in linux (If I remember correctly) caches the .torrent files in-> <user's home folder>/.config/qbittorrent/BT_backup
Add those files from qbt(windows) and choose the same save location as the in qbt(linux).
Note: If you download with one client some data and then use another client to download then the second client will do a "check" on all files of the torrent. This can be time consuming....