Adding tannins to fresh juice

Anyone on here make wine from the juice that comes in the buckets without skins? I'm looking to add tannins to some of my upcoming wines (juice will be in in two weeks) I'm not at the point of crushing my own grapes yet so I'm looking for ways to add tannins. On my counrty wines I used the powder form in the must but not sure if I can use it with juice. Any comments, concerns or ideas would be appreciated, Ken