An RC filter with a large C is likely all you need to filter out the DC/DC hash. Use a C with a low ESR (tantalum, ceramic, special low ESR electro.s) as this will be the main cause of propagated ripple. Of course the resistor will drop voltage proportional to current; but this is unlikely an issue.
So where's this spec? If you can state converter ripple, voltage, desired ripple (filtered), output current, output voltage tolerance (filtered), the actual values can be calculated.