Adding Stock image as secondary rom in multirom? I have a nexus 5, and root. I also use multirom application. I want to create a secondary rom from stock. However I cannot flash factory images hence need to make flashable zip out of it. How do I do it? What other method is recommended?
I'm assuming you have all the things necessary for MultiROM, i.e. modified TWRP recovery installed, MultiROM zip flashed, a custom kernel in Primary ROM having kexec-hardboot patch. 

If you're unfamiliar to the terms I used or the requirement is not fulfilled, see Huey's answer here or find the thread relevant to your device, on XDA forum or here.

Instructions:


Boot into TWRP and create a full nandroid backup of your current primary ROM  (my assumption is that the device is running the stock ROM).
On the default page of TWRP :


go to Advanced &rarr; MultiROM &rarr; Add ROM,
select the memory in Install to &rarr; Next,
select Backup &rarr; select your recent primary ROM backup &rarr; Swipe to Confirm, 
after flashing completes, do the reboot and choose your new secondary ROM in the boot menu. 



Done!



If you're running a custom ROM as primary, then take its Nandroid backup, flash the stock ROM as primary, follow my aforesaid instructions, and then restore the backup to come back to custom ROM as primary one.