The former Coventry striker has agreed a three-year contract subject to a medical and the relevant paperwork being completed to follow Antony Kay and Peter Clarke to the Galpharm Stadium.
Clark told the club's official website: "I'm absolutely thrilled to have Robbie Simpson moving to the club. He is a young, hungry player who wants to get better and that is exactly what I want.
"People who are looking at his recent goalscoring record need to realise that he has played 95% of his games out of position for Coventry.
"Robbie will give us a physical presence in attack; he works very hard and has a hammer of a right foot. I'm looking forward to working with him.
"I couldn't be happier with the three players that I have been able to bring into the club in the close season so far - Antony Kay, Peter Clarke and Robbie Simpson. All three players will give us a real presence and stature through the middle of the park."