You don't say what kind of K1 or why you want to do this. If it's a whitewater boat and you want the change to get a particular hull design as a C1, this is fairly common. You'll probably want to buy a commercial seat (check the Dagger website, or other manufacturers). You could also build a simple foam seat, but you'll want to see one that someone else has done to find out now it's done.

If it's a racing kayak or other specialty boat, then I think the other comments apply. I have a wildwater boat that started life as a C1 but which I paddle as a K1. In fact, the hull itself is just a Prijon K1 hull, shortened a bit to conform to C1 racing rules. The deck wings are also a bit wider, but this is rules oriented and cosmetic only. It doesn't change the basic hull. This works OK for me as I don't race it much and am pretty lousy at wildwater anyway. However the reverse, turning a K1 into a C1, would produce a boat which did not conform to C1 rules for wildwater.

A third possibility is that you have a relatively stable open cockpit kayak and want to try paddling with single blades. You can probably just improvise seats from cushions and try it out, but remember that the higher your center of gravity the less stable the boat will be.