Aftermarket subs can be acheived by several means. The first and easiest would be tap the line in off the stock amp and put your own amp and crossover in. Not sure what a "low end converter" is, some sort of subharmonic sythesizer? to give the feel of more bass? You can also ad a resitive network to pad the speaker level signal and impedence back to line level and then ad the amp and subs. I would take it into a reputable car stereo installer and see what they think they can do, maybe get some ideas. Avoid the the 16 yr olds at Best Buy or Circuit City.
You might even just want to change out the stock components with a better quality brand. What system do you have in there now? The HK or the basic?