Hi @Anonymous User
From my limited experience using the ITV Hub App on the nowtv boxes or nowtv stick i don't believe you have the option to "sign in" with your ITV Hub Account details and the only option you are given is entering your post code.
No, unfortunately you cannot. I use ITV Hub via my Freeview Play enabled TVs built in app.
http://www.itv.com/help/itv-hub#_where-can-i-use-a-hub-subscription