Scott, most recently a coach at Sale Sharks, worked with the Scotland A side last year and he admits he'll give his all to the cause.
"It's great to be back coaching and to continue my association with Scottish Rugby, which has meant a huge amount to me throughout my life," said the 38-year-old.
Ryan, former head coach at Bristol and Gloucester, won four caps for England at No.8 and will take on the role as forwards coach.
"I've responded to an invitation from Scott to assist the Scotland management team over the next 12 weeks," said Ryan.
"I will look to bring my experience to bear to give Scotland players the best opportunity to represent their country in the best way."
The duo will join existing defence coach Matt Taylor and specialist coaches Duncan Hodge and Massimo Cuttitta, and Johnson believes he's got the perfect blend.
"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."