Basically you are talking about a hull extension. A common rule of thumb is that you can get away with adding 10% of the boats LOA without needing to move running gear. But you should get an NA to design the attachment and GRP layup. Pick a yard and someone you are comfortable working with (know, trust etc), they will know some NA's who can provide the details. You may not be able to get a fixed price quote. Hence the trust aspect. It can get expensive - time adds up rapidly when doing that kind of work. But it's not rocket science and you need not spend a fortune. However, see below. Its likely more cost effective to buy something with what you want already in place: a different model boat.
For my 50' OA there are quite a lot that have had a 5' extension added. The original designer (Ed Monk) did drawings for some folks but I don't know whether everyone sought his advice. I considered it a few years ago, while the boat was in the USA. Two large yards who had done it on OA Mk1's previously told me it would run to about $120k to do it properly, by which they included a hull paint job in order to ensure the join was not really visible. The paint job was a big cost item though. The props, rudders etc were not going to be moved either. Although I think such an extension would be a huge benefit, I was not inclined to proceed at the time. One day, maybe.
