You certainly can do it, on both the front and back walls. You need to ditch the plastic fan holders and mount the fans directly to the case. There's a guy ("fieroboy" I think) at Ars who's done this and has a set of drill templates for locating the fans. It's really tight so you'd save yourself a lot of time if you used his templates. Give the search function at Ars a try, it was posted in the Case and Cooling forums last year or the year before. If the search doesn't work start a new thread about this, maybe somebody can point you in the right direction.