Doesn't have to be voluntary. Just use captured Russians for it. Same as we did to Germans (both German soldiers and civilians were used for forced labor). In particular, Ukraine has captured lots of non-Russians who fought for Russia. Typically no-one wants them back so they're worthless for trading for Ukrainians. Like everyone fighting for or otherwise supporting Russia they're criminals and executing them would be acceptable. Giving them the opportunity to work for their life is fine too.