OK, if your indash HU doesn't have a DSP menu setting in the menu, you don't have DSP.
If your car doesn't have a sub, and you want to add a sub, I would tap into the F door woofer wires coming out of the amp, and use those for a signal. Many amps will take speaker level signals in, and if yours won't, Line Output Converters are cheap.
If you have a voltmeter and you want to make SURE that whatever wires you find listed here are the right ones, fade the system all the way forward and turn the bass up all the way and turn the highs down all the way and then test the wires with your voltmmeter probes, with the meter set to AC voltage. The woofer wires will show more voltage, going up and down with the music, and when you think you've found the woofer wires, cut one. If one of your front woofers stops working, that's the right wire.
If this doesn't make any sense to you, or if you don't want to cut a wire, or it you don't have a voltmeter, please, please, please pay a pro to do this for you.
There's a great deal to learn here. If it's not making sense to you, you may lack some of the underpinnings of how audio gear works.
I'd go to here and read more:
http://forums.bimmerforums.com/forum...d.php?t=167293
www.bcae1.com