I imagine it would be difficult to sell a kit directly without significant changes. That's not to say it can't be done - but the kits are kind of like a game skeleton. You'll need to make it your own unique production to get anyone interested in it. Think of it in terms of value - anyone can get and use the kit, so it has no value. Thus, any value in your game is entirely from what you have added yourself. The more value you add to the kit, the more valuable it becomes to potential purchasers.
Sadly, a few new levels are not going to increase the value significantly.