The 35-year-old striker, who ended last season on loan at Scunthorpe, had been training with the League One club before he was diagnosed with testicular cancer in October.
Horsfield underwent surgery and was given the all-clear this week and Mullen would welcome the well-travelled forward back to the club, should he wish to continue his playing career.
"I'm absolutely delighted for him and his family, he's got an open invitation to come and train here any time," he said.