He played alongside Johnston, who was at York on loan from Castleford, for parts of last season.
Both players have signed two-year deals.
Coach Karl Harrison said: “Ben is someone we have been monitoring for a while.
“Cas rate the kid, but they didn’t feel he was going to make their side next season so they agreed to release him in the best interests of developing his career.
“He’s a Rob Burrow type player; small, with plenty of pace and very quick feet and we’re excited about his potential.
“Simon’s different in that he’s been around for a while at this level and he’s won a Grand Final.
“He’s an exceptional kicker, a very good passer of the ball and quicker than people give him credit for.
“He also knows how to run a gameplan, which is what we wanted.
“I’m not sure why he went to York, I know Mark Aston at Sheffield was devastated when he left them, but I do know he sees this as a big opportunity to get back on track.”