As it is, there's sub_pen_left and sub_pen_right, which the game puts next to eachother to make a full 2 pen bunker.
What you describe would (I think) be possible as you said, by cloning and giving it a new texture, after that you should go and edit the harbour layout and change the references to the original 2 sub pens to have, for example, sub_pen_1, sub_pen_2, sub_pen3 etc.
So, very possible, but quite a bit of work just for a few letters.
Or, if someone really wanted, they could just make one big bunker model and replace the stock bunkers with it, but I think there might be problems to get it fit to all harbours.