The 31-year-old will ride for the Doodson Motorsport Honda team alongside Tom Tunstall.
Harms impressed on a Honda CBR10000RR Fireblade in testing at Donington and during a shakedown, leading to an offer of a ride.
"I have been hungry to race in BSB for a couple of years now so I am very happy that Tom and Doodson Motorsport have given me this chance at the end of the season," Harms, a former rostrum finisher in the World Supersport series, told the official BSB website.
“Riding a Superbike is new for me, but I learn and adapt quickly and wish for a good performance to show my skills for a seat in 2013."
Tunstall said: “We're very pleased to welcome Robbin on board. It was good to work with him at the Donington test and we see the Cadwell round as an extension of that test really.
“To go to Cadwell Park for his first race isn't an easy prospect. It's a very tight and technical circuit, certainly a one-off - there's nowhere else like it in the world - and a real challenge on a Superbike especially when you're still getting a feel for the bike.
“The following round at Donington will feel like racing on a motorway by comparison.
“He's got good experience of the circuits for the final four races including podiums in World Supersport, so he can build from Cadwell and start to show his capability as he gains more experience.”