I know lot's of fisherman (and women) that do quite a lot of fishing but don't use a livewell. I've seen some Offshore models on lakes and I'd bet most Marlin owners have never caught a marlin. I suppose they could have made it a standard feature, but then the boat would have been more expensive to begin with. I'm sure the person who ordered the boat appreciated the fact that they didn't have to pay for something they didn't want - I doubt they cared much what some second or third owner wanted.
I'm sure anything is possible as far as adding it in the stock location. Finding out if the parts are available by asking your closest dealer would be a good first step. The plumbing/pump part is definitely doable - it's whether the livewell tub, itself, was custom and still available. Otherwise, you might look into alternate types of stand-a-lone livewells - there's a recent thread about this, too.