I have no idea how to do that in Calibre; I guess you will have to run your file through a proper OCR program like ABBYY Finereader (or one of the free OCR applications based on Tesseract). I don't know which of them process djvu, though; perhaps you will have to save the individual pages as images first.
Anyway, no matter how well your OCR turns out, you will always have to manually proofread the result.
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