The 24-year-old right-back is set to complete a move to the Premier League new bots in the next 24 hours, following striker Kevin Doyle (Reading) and midfielder Andrew Surman (Southampton) to Molineux.
And he will take McCarthy's summer spending in total to approximately £12million.
Halford spent last season on loan at Sheffield United and will provide competition for Kevin Foley, who was Wolves' player of the year last season.
Halford made the £3million move from Reading to Sunderland two years ago when the Wearsiders were managed by Roy Keane - but had restricted opportunities at the Stadium Of Light.
He had spells on loan at Charlton and then the Blades.