Add an extra line to addresses in Contacts in iOS?
As you need to do a shift+enter (also called 'soft return') to do it (because 'enter' acts like 'tab'), you can't on iOS. According to this recent post Zoagli points it in his answer you can't do it on iOS.

Alternatively you can try to add the soft return on a computer via Contact.app (on a mac), or via the web interface of your contact manager (Google contacts…)

PS iCloud on the web doesn't support soft return.