SEEDING SOMETHING I HAVE ALREADY

I have asked this before but i could find it,so sorry if this a dupe post.
"downloading from 1 site site,but seeding it on another"
I have a a release,which i have either downloaded from site A,or have fxp it over,now with utorrent,i would just download the .torrent file from site B ( were i want to build ratio buffer) it would rehash,maybe start at like 99.9% then just seed,ive tried a few times,but it just wants to download the entire thing again,if someone have time could they put a dummies guide on how to it,you w ill be forever in my thanks