You need to alter the table like add the Maybe value to the ENUM:
ALTER TABLE tablename MODIFY COLUMN columnname ENUM('Yes','No','Maybe');
Then change NULLs to Maybes:
UPDATE tablename SET columnname='Maybe' WHERE columnname IS NULL;
And finally disallow NULLs in the column:
ALTER TABLE tablename MODIFY COLUMN columnname ENUM('Yes','No','Maybe') NOT NULL DEFAULT 'Maybe';
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