You can't really take on anyone else's karma; deities can do that, humans can't. (Old-fashioned ccult authors like to say that when Jesus said "your sins are forgiven you," what he was doing was literally taking the person's karma on himself.) What you can do is take on a larger than usual share of the collective karma that our species has piled up, thus clearing the air for everyone. As far as I know, there's no problem with that -- well, except that the person who does this can expect to have dumpsters of misery unloaded on them.