We can work backwards so to speak:

    f[r_, s_] := Union[Select[
      FromDigits /@ Join @@ Permutations /@ IntegerPartitions[s, Ceiling[Log10[r]], Range[0, 9]], 
      LessThan[r]
    ]]

Example:

    f[1000, 6]

>     {6, 15, 24, 33, 42, 51, 60, 105, 114, 123, 132, 141, 150, 204, 213, 222, 231, 240, 303, 312, 321, 330, 402, 411, 420, 501, 510, 600}

    Total[IntegerDigits[f[1000, 6]], {2}]

>     {6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6}

Larger example:

    vals = f[10^12, 100]; // AbsoluteTiming

>     {0.069749, Null}

    Length[vals]

>     75582

Verify:

    Total[IntegerDigits[vals], {2}] // Union

>     {100}