technically you can, with a caveat.. on 2.1 systems the subwoofer signal is pulled from the stereo input meaning the sub will fire when the speakers do. this might sound odd if you're hobbling together a 4.1/5.1/6.1/7.1 system. you can turn gain to low and volume to minimum to try and counter this but i cannot guarantee it will sound the same as a 5.1 system.
they did make a 5.1 promedia set. not sure its still made though.
i had a promedia 2.1 system for a few days before i returned it. i liked the sound but the satellites were droned out by the powerful subwoofer. i did end up staying with klipsch speakers though. given how you want to upgrade, you might want to consider the klipsch quintet speaker system, getting a subwoofer and a receiver. much, much nicer than the promedia if you have the money for it that is...
or, a cheaper home theater set perhaps.. monoprice 10565/ energy 5.1 classic take and a receiver.
just a few ideas.