What is the maximum temperature the magnet can withstand before being damaged / losing magnetic properties? If the magnet can withstand heat then a silver solder or braze may be an option. An epoxy optimised for nickel - an awkward metal to bond IIRC - might be an option too.
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We would need to know the type of nickel plating used on these components. If you used sulfamate or electroless nickel, soldering would be the best way to go.