In my opinion you should be filtering the deepest bass from these speakers when adding a subwoofer. To do this you will need a "crossover" in the system somewhere. The subwoofer may have one internally the will filter the signal in this manner. Look for a subwoofer that has both line level inputs AND outputs. Then read the product detail to see if the outputs are filtered.
Otherwise, you will need to add an external crossover. Usually the least expensive units are in the $69-99 range.
If you wanted to forego the speaker filtering for now, you should be able to find a "splitter" for the outputs of your sound card. Run one leg into the subwoofer and one leg into the speakers. The subwoofer should have both L/R inputs for this to work. Also, I'd send an email to the sound card developers just to verify adding a splitter to its output is ok.