只需在内部进行计算 group_concat() :

set @word = 'hello';

with recursive cte as (
    select @word as word, left(@word, 1) as val, 1 as idx
    union all
    select word, substring(word, idx + 1, 1), idx + 1 
    from cte 
    where idx < char_length(word)
)
select group_concat(ascii(val) + @add order by idx separator '') ascii_word from cte