I can only think of one example of people being sent to a penal legion in Black Library fiction, and that's in
For the Emperor, by Sandy Mitchell. A group of Guardsmen get into a scuffle, and while most of them are given a slap on the wrist, the more severe agitators (the ones who actually killed people) are sentenced to service in a penal legion. The Inquisitor in that novel also has an ex-commissar she recruited from a penal legion as part of her retinue. (I think that guy was from
For the Emperor. I might be wrong on that...)
You might want to check out the
Last Chancers series, by Gav Thorpe, for more information.