Manager Lee Clark told the club's official website: "We have monitored Nathan for a long time and have had him watched by a number of scouts on many occasions.
"I have been to watch him myself along with the coaching staff and it is clear he has a lot of good qualities. He has real pace and can play anywhere across the front line, whether that is as a striker or on either wing.
"Dean needs to have a good run of games from now until the end of our season to cement his place in the World Cup squad so he has a huge incentive to come into Europe and play well.
"He has a lot of experience behind him and possess a lot of qualities. He is a very fit lad, a real athlete and has very good pace. He should be a very big asset for us."