Navi, I'd like to hear David's or DocV's opinion, but those sentences sound bad to me because of the change of subject. I think it'd be more appropriate to say:
1a) Our truck was stolen to be used in a bank robbery.
2a) Somebody stole our truck to use it in a bank robbery.
3a) They were kidnapped to be exploited as slaves.
4a) The traffickers kidnapped them to exploit them as slaves.