it can be done..but..its not for anyone without serious experience to do..a trailers build is actually a very well thought out process to arrive at a trailer that will haul its intended load safely and reliably...what kind of trailer,whats its factory sizing(including frame dimensions and weight rating), and what are you trying to haul with it?..most common way to do it is to remove the tongue, add new frame rails under the existing rails from the axles forward, while extending the bed to the legth you need(which may result in moving axles,adding axles)..it can be done for sure..but it needs to be very very well thought out....