A Jasper Big Block DemoN install is the same as a small block install, but remember to build the final image with the "Jasper 16MB" motherboard type selected in J-Runner.
I am pretty sure you can extract the vital files from your NAND image (KV (encrypted), SMC and SMC Config) and inject them in to a Jasper 16MB donor NAND, but this isn't really a task for a n00b because you need to know which blocks to write those files to and it takes quite a bit of time to do.
This is how creating a NAND image without a full NAND dump works - XeBuild can build an image with only the KV, SMC and SMC Config available, but the CPU key is required.
If you look at an XeBuild image log, you will see that XeBuild checks the "XeBuild\Data" folder before looking at the NAND image for those files.