Macadam has made 15 appearances in the First Division North this season, and one in the Lancashire Challenge Trophy.
"We are delighted to have secured Harvey," said manager Michael Clegg.
"We have to Thank Andy Harrison at Nelson for this. I have a good relationship with him and he made me aware a few weeks back of the talent he had on his hands at Nelson.
"Harvey has come in training and has been excellent. A young lad with ability, great physique and humble personality.
"We are really grateful to Nelson FC for allowing Harvey to continue his development with us.
"We are looking forward to working with him."
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