If the 3.5mm cable for your powered 2.1 PC speaker set is long enough, you can just splice in the jack (see pic). If it is not long enough, buy yourself a 3.5mm male extension cable and just snip the end off of it.
I think I'm going to add headphone jacks for the line coming from PC Audio Out and the Speaker output. Do you think thats a bad idea?
I think you are suggesting that you want to add headphone jacks on each end to keep it clean looking? If so, that's fine, it will sound the same either way.
One note...sometimes the PC speaker line out has different impedance level than the headphone jacks, so just test your headphones in the PC out jack first to be sure they will provide enough power (sound good). Sometimes the jacks are basically the same and sometimes they have different output specs (straight line out or amplified for headphones). Just plug your headphones in and listen to see if they sound o.k.