The 25-year-old, formerly of Ulster, Bristol and Yorkshire Carnegie, returns to Castle Park for a second spell after finishing as the club’s top points scorer in 2011/12.
McIlwaine will combine his playing and studying commitments at Leeds Beckett University with Knights before joining the club on a full time basis.
Director of rugby Clive Griffiths told Knights’ official website: “We know Dave’s well suited to Castle Park, he’s been here before, knows the club inside out and when I got in touch with him he had no hesitation in coming down.
“Dave has to combine his training with us and Leeds Beckett University who are still in the BUCS competition and play Hartpury College a week on Wednesday in the national quarter finals.
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“Once his commitments with the University have concluded he will be with us on a full time basis but in the meantime he will balance the two.
“He’s got a good kicking game and is not short of a yard or two.
“I know he’ll be well-suited to our game plan and style of play and I just hope he can bring all his experience to the table and help us out.”