mid-career hiring; joining a company mid-career
Refers to the act of joining a company as an experienced hire, not as a new graduate. Often used in contrast to 新卒入社 (new graduate hiring).
He joined the company mid-career and is making use of his previous work experience.
There is training for mid-career hires.
新卒入社 refers to joining a company as a new graduate, while 中途入社 is for experienced hires.
Compound of 中途 (midway, mid-career) and 入社 (joining a company).