if you can build a Crack, you can build a dongle like anetode posted easily enough to suit your needs, and play around with differing resistances.
yes, just add the resistors in series in each channel and leave ground connected as normal without any series resistance. if you want to play around with adding output impedance balanced, cut the resistance in half of total desired resistance since both phases of a single channel are in series with one another across the driver. if you want a total of 120 ohms, add a 60 ohm in series with each of the 4 wires.
make sure you ohm it out when finished for any possible shorts in a tight fitting plug.
