Ryan has previously enjoyed stints as head coach at Bristol and Gloucester. He was recently appointed as a consultant at Newport Gwent Dragons, alongside his punditry work with Sky Sports.
"I've responded to an invitation from Scott to assist the Scotland management team over the next 12 weeks," he said.
"I will look to bring my experience to bear to give Scotland players the best opportunity to represent their country in the best way."
Johnson was pleased to finalise his coaching staff for the Championship, with Scotland desperate to improve on last year's disappointing showing.
"I'm delighted we have secured the services of Dean and Stevie for this year's Championship," said Johnson.
"To get people of their calibre mid-season is a real coup.
"Dean is a sound, knowledgeable, mature rugby man and Stevie, with his background of having played 11 times for Scotland, knows what it means to wear the jersey."
Glasgow forwards coach Shade Munro will head up Scotland A, where he will be assisted by record caps holder Scott Murray and former international stand-off Duncan Hodge.