How can I install a supersu root zip on a system.img to create a pre-rooted system?

I'm looking to create a pre-rooted system.img for a specific device. running Marshmellow (6.0) on ARM. I have a SuperSU zip for marshmellow and the compiled system.img for my device. OK. I know you cant flash a zip to a .img - but, can i decompile the img, add the resources, and recompile? I specifically want to know - 1. Most importantly, is this possible...? 2. If yes, What files do I need from the zip? 3. How do I decompile the .img while not losing any important data? 4. Where in the decompiled .img do I put the files from the zip? 6. How do I create symlinks and set the permissions for the new files? 5. How do I recompile the .img maintaining all linux permissions etc.? Note, working environment is preferably windows. (although if I can get detailed instructions for Ubuntu, that'd be great too.) This question begs quite a detailed answer, and I'll be most grateful for any assistance on the subject, even incomplete. The device in question happens to be a VN220LTE, but I assume that the approach would be generic.