It's probably very hard to switch from offense to defense, but that is what 6th round rookie FB Korey Hall is trying to do. It sortof worked out with ex-Packers FB Vonta Leach, who the Packers converted from college linebacker to NFL fullback, and Houston made him a millionaire this offseason. Leach had a lot of playing time in 2005 for the Packers and made some fantastic lead blocks, but seemed to miss as many blocks as he made and he ended up getting released by the Packers in September 2006.
If Hall is fantastic on special teams this preseason, then he makes the roster. However it seems more likely that he will be released and then hopefully he will sign onto the practice squad so he can spend a full season learning how to play fullback.