How do i use a lockscreen from another rom
Unless the lock screen is some kind of 3rd party app (like GO Locker), then I believe you would have to modify the ROM yourself, and port the lock screen over manually, into your 'new' ROM. - In which case you might be better off asking for advice over at XDA, or another developer forum. 

If it is some kind of 3rd party app, chances are you can download &amp; install it from the Play store, or download the APK online from the developer's website. 

EDIT: Rereading your question it's occured to me you might actually be asking how to port over the lock screen (IE: which files to modify in the ROM) - in which case, you should probably go to XDA (see above link) and ask for advice.