Think of piggies as tender bedding plants - they really need to be indoors to be protected against the cold nights and frost until the nights are much warmer. If they are youngsters or have never lived outside, then definitely they need to be indoors for a while longer.
Heat pads and covers for the hutch are a good idea for outdoor hutches to keep them warmer and drier.
Do you have somewhere that a hutch could be moved into when the weather is really cold (October - April usually). A garage or shed make good locations for an outdoor hutch to be moved into for overwintering.