I was converting a NB Extract Kit to a full boil a few months back and asked NB. Here was their reply:
The general rule of thumb that I would follow for doing one of our extract kits as a full boil would be to add all of the extract at the beginning (no late extract additions), and to follow the hop schedule on the All Grain kit instructions.
The instructions for each kit are available on the kit page, under the additional information tab.
Let me know if that makes sense.