How to create a flashable zip using factory images?

[AC Question]

I have Nexus 5 which is rooted, and also use multirom. I want to flash a Stock Image as secondary ROM using multirom. However I cannot flash the factory images directly. It demands flashable zip. So how do I do it? I do not want to flash the factory images directly to the phone as it would wipe user data as well as other Roms that I have installed.