First - all subwoofers ARE passive, as all speakers.
To make one speaker active it means adding to it amplification from the inside, as externaly it the usual way.
I guess you want to use 2-way amplification: splitting the frequency spectrum between 2 regions (speakers or speaker cabinet).
The way to do it involves using 1 amplifier (or 1 ch of a stereo one) for each frequency range. The sound chain goes like this:
1. the stereo signal is entering a crossover (device to splitt the spectrum).
2. from it's outputs (normally High and LOW) you connect to a diffrent amplifier, one for the High's and one for the Low's.
3. now you have 2 amplified signals, one for each frequency range.
connect the Low (bass, Sub) amp to the Subwoofer. connect the High amp to the remaining Mid/Hi/ Tweeter speakers.
Now.. if you want to make apassive Sub into an active one - only - as in stereo system, you'll have to let it use it's own amp, either internal or external. If you dont have separate feed for the SUB (ie, signal that carries only the low frequency), you need to do it by yourself or buy crossover filter, which does just that.
I hope this answer your needs. if not - please be more precise as what you want to accomplish.
Crossover Network Design Formulas & Calculator
http://xsspl.tripod.com/Audio/CrosFrm3.htm