You mentioned in a previous post that your kit was originally a rowboat. It''s very likely that the designer never allowed for a tiller in the layout of the seats.
To some degree, you could shorten the tiller (all it does is provide leverage). In theory, if you (and it) had enough strength, the tiller could be 4 inches long. In reality, I''m guessing that with a tiller as short as practible, you will still not find the rear seat useable.
My mind conjurs up a complicated series of levers, sheaves (pulleys), and non-stretch lines to transfer the steering control to an alternate position, but I doubt it would allow you to control the boat very well.
Sure would be nice if there were a picture of the boat available on the web to view.