depends what you mean by "adding" sounds.
by the nature of your 002, you have the ability to record with 8 analog inputs, and 8 digital plus the spdif. you can't add anything more than that - 18inputs.
if you've used up the 4 preamps built in to the 002 and want to use more mics, you can explore various options. if all you need is another 4 mics, you could find 4 pres with analog outs and be set. but if you're recording drums lets say, and you need more than 8 mics, you're going to need to go the optical route. depending on your budget there's lots of options for this as well. behringer makes an 8 channel optical unit (ADA8000) and then there's mroe expensive options like the Digimax and more. Hooking them up optically is fairly easy. just get an optical cable and you're connected.
I've used the digimax a few times and its pretty convieniet. I've also tried
the focusrite ISA428 and found that i much rpefer that. but there's obviously a price difference.
So to sum up (sorry i know i'm rambling)... you can't add more inputs to the 002 than what its capable of recieving. but if you want to use more than the 4 mic pres they give you, it can be done and you can get some great results out of it. it all depends on your budget
