If you are referring to Penitent Engines:
Any repentant heretic may be sentenced to be condemned to a Penitent Engine. It is used when arco-flagellation is deemed to be too lenient a punishment. I would think that a Sister of Battle being so sentenced would be amazingly rare.
I was being cheeky, but : http://warhammer40k....Penitent_Engine
It does seem to imply that sisters are more than capable of suiting themselves up into this thing. After all the penance can be for something even imagined.
That being said, I took more consideration with the argument that bikes would be costly to throw away effectively being used on repentia, considered they are willing to dump thrones on huge walkers for a single pentitant pilot.