Lowes, who was right-hand man to Steve McNamara in England’s Test series triumph over New Zealand last autumn, left Carnegie just under two years ago to take charge of his former Super League club but resigned for personal reasons.
His return to Headingley is a timely boost for Carnegie, who launch their Championship play-off campaign on Sunday against Doncaster at Headingley.
The aggregate winners over the two legs will play Bristol or Bedford in the two-legged final to earn a place in the Aviva Premiership next season.
Carnegie head coach Bryan Redpath said: “Jimmy is obviously someone who knows this group of players very well and the club as a whole.
“Once we learnt of his availability, we approached him about the possibility of joining the backroom team for the play-offs, similar to the way Alan Tait has been working with us in recent weeks.
“Jimmy lives locally and was keen to help out where he can and we are looking forward to having him involved from when training begins this week on Tuesday morning.”