A wide variety of SQL operators can be applied to field data with the SQL string. This string can contain any combination of SQL expressions that are recognized by Oracle as valid for the VALUES clause of an INSERT statement. In general, any SQL function that returns a single value can be used.
So perhaps:
CASE WHEN :your_col = 9999 THEN 9999
ELSE 12*trunc(:your_col/100) + mod(:your_col,100)
END