in normal pc 2.1 systems both speakers plug into the subwoofer which in turn plugs into the computer. the subwoofer also features the power connection.
with a 2.0 you are plugging directly into the computer. depending on what you have.. one of the speakers might have a power connection they share.
would it work? probably as long as you didnt overpower the speakers.
would i do it? no. i myself wouldnt risk it. if you want a 2.1 just buy a 2.1 system and return the old one if its that new.