Yes, it is possible to create a small circuit to accomplish what you're trying to do, but it usually takes some trial and error testing, combined with the abililty to solder and the specs of the subwoofer amplifier with regard to whether it is a bridged output and its power rating.
Your best bet would be to use a set of 5.1 speakers, thus enabling the sub out jack in that mode. Then you can tap the line-level output to feed the BFD and Vel.
If you simply used your 2.1 set of speakers as mains left/right and then added a powered center channel speaker, you could enable 5.1 mode to make the center/sub line out active.
brucek