Hmm...
So you are trying to seed torrents, downloaded from BxT, with files you got from another source? If so, then it seems your method is correct. Right click the BxT torrent, point uTorrent to the folder in which you've placed those other files, force a recheck and if you've got all the right files it will show 100%. Then, depending on your preferred options, you might have right click on force start/upload.
Is this your situation, I couldn't quite make it out from your description.