Knott has made 18 appearances and scored twice since arriving from the Black Cats earlier this season.
"It's huge to be able to keep hold of him," manager Gareth Ainsworth told the club's official website. "He's at the heart of a lot of our attacking plays and he's got abilities above League Two.
"Billy's a big part of the plans. I've always said I want to keep my best players and he's right up there.
"He needs work on certain aspects of his game but he's getting a lot of first-team experience here.
"Billy's great to work with so I'm really pleased that he's staying. Thank you to Sunderland again for allowing is to keep him for another couple of months."
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