how to add a speaker as subwoofer?

Hello, I'm a newbie!
Does anyone have the solution to my problem..
<pre>
+------+
| |
| PC |stereo line out
| +===========++
+------+ || ?????
++==++= = = = = = = = = = = ++
|| ||
speaker A || ||
+----------++-----------+ +---------++-------+
| small active speaker | | single channel |
+-----------------------+ | 5.25" driver |
/ L \ / R \ | amplified |
/_____\ /_____\ |? How to make ?|
|? this speaker ?|
|? to a subwoofer ?|
+------------------+
speaker B
</pre>

Is it possible to turn "speaker B" into a subwoofer?
If so, how?
If it's not possible, I think "speaker B" here can be used as a bass extender from pc's output, but I don't have a clue how?
Anyone can help?

thanks
andriwas