Who said kidnapping?
Having a fake Sr Lucy would be as simple as her Mother Superior telling her she can't talk with anyone or leave her room without permission. Then Vatican officials bring out an imposter nun to pal around with +JPII and give "interviews". Sr Lucy, being a saint, would comply with any and all orders, and she wouldn't break the secret that she had never left the convent. It's that simple.
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Notice also that subsequent interviews with the real Sr Lucy, where she said that the 1980s consecration wasn't totally acceptable, and the other interview where she used the phrase "diabolical disorientation" were all done at the convent.