With the extension McNulty has also been allowed by Barnsley to play in the weekend's FA Cup match against Accrington Stanley.
The club's manager Ronnie Moore has admitted his delight at securing the extension for another two months.
"I'm really pleased to be able to keep Jim here until January," he told the club's official website.
"He has settled in very quickly and I've been happy with his performances. He's a strong defender who uses the ball well and he has looked comfortable in the middle of defence or at left-back, so he gives us options."
Tranmere are currently in the League One relegation zone.