Addy, 25, is predominantly a loose-forward or stand-off and has more than 150 first team appearances under his belt.
Born in Pontefract, but raised in New Zealand, 23-year-old Clark is a former Junior Kiwis international who followed in his father Trevor’s footsteps by playing for the Bulls
A prop-forward previously with Doncaster, he came through the ranks at Penrith Panthers in the NRL and scored a try in the under-20s Holden Cup final in 2013.
Both players have signed a one-year deal with the Robins, with options for 2018 and 2019 subject to the club’s league status.
Hull KR’s head of rugby Jamie Peacock said: “We’re delighted to welcome both Danny and Mitch to the club for 2017.
“We’ve been looking to increase the depth of the squad and both players have plenty to offer.
“Although they are still in their early to mid-20s, they have plenty of experience for their age, have a good knowledge of the Championship and they are both hungry to come to Hull KR and make an impression.”
Peacock added: “Danny is able to cover a number of positions to a high standard whilst Mitch is a big prop with bags of potential.
“He’s got time on his side until he hits his peak as a front-rower, but he’s a strong ball carrier who will provide the team with some real impact and go forward.”
Peacock is delighted both players stuck to their decision to join Rovers, despite relegation from Super League this year.
He said: “Although we had initially agreed contracts with them earlier this season, both Danny and Mitch have shown they are up for the fight next year and are committed to helping us push for promotion back up to Super League in 2017.”
Addy is expected to feature for Scotland in their Four Nations clash with Australia at Hull KR’s Lightstream Stadium, Craven Park, on Friday, October 28.