The 2nd Division club announced Jong's signature earlier this month, with Roberts adding further AFL experience to the local club's squad.
Roberts, a member of the Bulldogs' 2016 premiership outfit, played 51 games in the blue, white and red across eight seasons at Whitten Oval.
Following seasons at Williamstown and Port Melbourne in the VFL after his delisting at the Dogs, Roberts is set for a stint at the grassroots level.
“I decided to step away from VFL footy and I probably had a few options… Mulgrave stood out and I'm very happy to be on board,” Fletcher said in a club statement.
“I know a couple of the boys and it hasn't been an easy period for the club with COVID and then the building redevelopment, so it makes this a really exciting time to get involved now that things are rolling.
“The list is very young and talented so hopefully some of my experience comes in handy. I really look forward to working with the young guys and the Under 19s coming through. It'll be great to play with Lin again too.”
Roberts averaged 7.2 disposals, 3.3 marks and 1.2 goals per game for Port Melbourne this year from 16 games.
Mulgrave managed an eighth-placed finish this year in the EFL's third-tier competition.