Probably to use a list comprehension:

    suffix_str_lst = [x + "_suffix" for x in str_lst]

If you are okay with a generator instead of a list, then you can flip the square brackets to parentheses and then you do not need to hold the whole thing in memory at once.

EDIT: You can add a filter in a similar way:

    suffix_str_list = [x for x in str_list if x.startswith("meep")]