You can't "impeach" somebody who's not holding a public office. However, HE COULD BE CONVICTED OF A CRIME just like anyone else, that incudes indictment, trial, judgment and punishment.
I would have to do some research; however, I'm fairly certain that I've read opinions that said it was possible to impeach a former president for misdeeds that occurred while in office (I'm fairly certain that it was commented on when the Marc Rich pardon scandal first became public).