Vyxal j, 10 bytes

×ẆvwyvkaYΠ

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×Ẇ         # Split on asterisks, keeping the delimiter
  vw       # Wrap each in a list
    y      # Uninterleave, leaving the words and the asterisks
     vka   # Replace each asterisk with the lowercase alphabet
        Y  # Interleave
         Π # Take the cartesian product (strings in lists treated normally)