i'm asking if i can use the mixer instead of buying a sound card
so I am correct. I didn't want to assume or answer before I was sure of your question.
having a great mixer is a nice asset, however you are skipping the quality of the sound to be able to change it later. traditionally speaking you'd want to get the source of the audio higher with a better sound card first before adding on the additional expense for a mixer. perhaps ask yourself - which has more of your energy? producing or dj'ing - and perhaps that could help you decide on your question. perhaps the real question is on a sound test, how much different a stock soundcard that you computers comes with vs. a real sound card. as Themis mentions you technically already have one.
It would be completely unorthodox, but sure you could do it. who's to say you may fall into your own custom sound that way?
Having a mixing also gives you the option to manipulate sound afterwards and also serves as a dull purpose if your goal was also to obtain one for djing purposes. you can get 24-bit/48kHz sound cards for like $80 now as well so even if you did annie up for a great mixer first you could also add a cheap sound card later and have the best of both worlds.
this was actually an amazing sound card for the price I got years ago, and actually used for my live set in moscow. I think back back when I bought it, it was around $120. now it's less than half of what I paid.
Lexicon Alpha - - 24-bit/48kHz $49http://www.sweetwater.com/store/detail/Alpha?adpos=1o2&creative=55225946401&device=c&matchtype=&network=g&gclid=CjwKEAjwotmoBRCc6LWd2ZnkuBYSJACyt2quFOEhtgAGbDPhpJOE3G4pjbK96Hby1FjDOTQqSTZYhRoC2pjw_wcB
also as mentioend you wouldn't need a dj mixer for a sound card for your computer. I assume you want to use the effects on the dj mixer or something? just really depends on the set up and goal. producing vs. dj. producing you'd want to get at least an i5 speed, then good sound card, then speakers, then plug in's or keyboards, then add an analog mixer.
you asking about keeping stock surrounding pc, getting a dj mix, so the order doesn't make sense really but everyone has their own way.