Yes, you can add a lifetime extension kit later if you would like. However, it is much easier to add it during assembly of the original shed. First of all, when you build the shed you need to put it on concrete or build a wooden base or platform. The instructions show you how to do that. If you intend on adding an extension later simply keep that in mind when you build the foundation or pour your concrete. You'll need to simply add the length of your platform/base/concrete slab by 30" or 60" exactly depending on the size of extension you choose.
Adding an extension involves removing the back two roof panels and unsnapping the edge of the floor. It's really not as bad as you might think. I'm not very handy and it's a peice of cake for me. Feel free to contact us with question.