The controversial move comes almost four years after McCormick caused the deaths of brothers Arron and Ben Peak, aged 10 and eight respectively, while driving with excess alcohol in a crash on the M6 in Staffordshire.
The former Pilgrims stopper was sentenced to seven years and four months in prison in October 2008 after he admitted to the charge, but is set to be released after serving half of the original figure.
McCormick has already been put through his paces having trained with Swindon since January while on day release. He will join Paolo Di Canio's squad on their pre-season tour of Italy.
Swindon chairman Jeremy Wray told BBC Wiltshire: "Our sympathy is with the family of the victims - that will never change. There's been a complete tragedy and lives have been destroyed.
"This guy has done his time in prison. He wants to come back and give something back to society. The best way we can do that is to rehabilitate, and that's the role Swindon can play. It's a benefit we've thought about in the long-term. It's going to be a big debate and I'm not frightened of it."
Wray continued: "We thought long and hard about it, and it has to be made clear that everybody's thoughts can only go to the family of those two young boys and their father who was badly injured.
"Tragedy is a word used too often, but this was a tragedy and it can never be changed.
"He (McCormick) will live with what has happened every day of his life but he has the chance to give something back, to show the tragedies of drink-driving."
He added: "As a club we did not go courting this - the opportunity was presented to us after I was contacted by his agent and he will spend pre-season training here at Swindon. We have had to look at it in a reasonable way and look to produce a positive outcome in horrible circumstances."
McCormick's contract with Argyle was cancelled by "mutual consent" a month after the crash.
Arron and Ben were travelling to Silverstone racetrack for a dream day out with their father Phil Peak and three friends when their Toyota Previa people carrier was hit by McCormick's powerful black Range Rover on the M6 at 5.44am on June 7, 2008.
The footballer was returning from former team-mate David Norris's wedding in Bolton at the time.
He was seen with his footballer friends drinking beer and downing shots of the spirit Sambuca at the wedding reception.
Stoke Crown Court, where McCormick admitted death by dangerous driving and drink driving, was told police investigators estimated his Range Rover was travelling at around 97mph moments before the crash.
Mr Peak, from Partington in Manchester, was at the wheel of the Toyota and suffered serious injuries that left him in a wheelchair.
The boys' mother, Amanda Peak, told the Sun that Swindon's decision "was like a kick in the stomach".
"Swindon might be a family club but now they are hiring a man who has torn my family apart," she told the newspaper.